So you need something. Maybe you need your equipment repaired, maybe life will become so much easier if you have a certain piece of equipment, maybe you need the next item in a Chain of Deals or perhaps you're outright looking for the next Plot Coupon. After much effort you track down the only source of this rare good or service (or perhaps try and fail to procure it from other sources) ready for yet another quest or to pay through the nose in exchange...and they're astounded you're so desperate to get your hands on something so easy to get hold of. Occasionally the trope is downplayed, with the item in question being hard to obtain, but granting a high yield (for example a rare ore which you can dig up tonnes of after finding your way to an isolated mine) or something minor compared to the rest of what you get (for example, requiring just a single coin made of a particular alloy or a special magic item in a massive horde) making just one a trivial reward.
This trope is a close relative of both We Have Those Too and Worthless Yellow Rocks. Not only does the person you're addressing have what you're looking for, but they're amazed you had to expend so much effort to find it (or maybe they're aware it's not easy for most people to find, but they have the means to get it easily). In a Video Game this might be used to lampshade a Commonplace Rare. Contrast Bulk Buy Only for when the protagonists have more of something than they want.
Remember that someone simply giving away things they have a large amount of doesn't count; it must also be what someone else is searching for. Also known as Shaggy Dog MacGuffin.
In the first Tenkaichi Budokai in Dragon Ball, one of the finalists is Nam, who wants to use the prize money to buy water for his drought-stricken village. After he's defeated by Goku, Jackie Chun gives him a capsule and tells him water is so plentiful in the region that he doesn't need to buy any at all: he can just fill the capsule at a nearby spring.
A story of Brazilian comic Monica's Gang has the title character going through lots of ordeals in search for a missing jigsaw puzzle piece. Then upon seeing the puzzle her father tells on how the woman on the puzzle was related to them, and hid that piece from the puzzle because it depicted her ugly nose... and then the father reveals all those missing pieces hidden in a drawer at their own house!
Gold was so common on Pre CrisisKrypton that it was considered a cheap metal. When Jor-El first tried building a spaceshipnote (Not the little one that would eventually carry baby Kal-El):, he was mocked for using something as heavy as gold; but the ship used anti-gravity, so the weight didn't matter. He had made it out of gold as a cost-saving measure.
A Storm of Swords: Daenarys tries to buy off a mercenary company and offers a casket of wine as a goodwill gift. The mercenary captain complains that this is not enough and demands a whole wagon of caskets instead, to which Daenarys obliges. The captain wasn't aware that Daenarys *did* just loot an entire city famous for (among other things) its wine.
In the Myth Adventures books, Aahz tells Skeeve that things that are rare in one dimension are commonplace in another, and so a skilled enough magician could make a small fortune selling cheap trinkets at marked up prices. the biggest offenders are the Deveels, who've become a race of traders supreme thanks to this trope.
The Counterweight Continent in Discworld has a rather large concentration of gold compared to the Ankh-Morpork area. From a Counterweight perspective, Twoflower's Luggage filled of treasure is an appropriate amount to cover his traveling expenses. From Rincewind's perspective it would probably buy a kingdom.
And when Rincewind first encountered Twoflower in The Broken Drum and saw his Luggage made of sapient pearwood, he estimated it was worth more than the drinks, the bar and all its inhabitants combined. In Interesting Times we learn that, while they're not common, many Counterweight nobles own one.
In The Burglar Who Liked to Quote Kipling, Bernie Rhodenbarr is hired to steal what he is told is a valuable, one-of-a-kind book. After he and several other people scheme and counterscheme for possession of the book, Bernie discovers that there are many other copies in the possession of the man who hired Bernie in the first place. The man has been selling the books as one-of-a-kind and is trying to prevent a powerful, short-tempered buyer from finding out that he's been conned.
On Beta Colony, wood is very rare, so Cordelia is shocked when its so common on Barrayar
While in reverse, Aral tells a story about how one of his Barrayarn ancestors invested in gems (rare on Barrayar) but ended up losing the family fortune because they were much more common in the rest of the nexus (due to synthetics).
In the Cordwainer Smith story On the Gem Planet, gems are ridiculously common and essentially worthless on Pontoppidan (the eponymous planet), but dirt is rare and precious.
In Triplanetary, the first part of the Lensman series, the Nevians come up with a way to transmute matter into (tremendous amounts of) energy, but the problem is that it needs iron, which is incredibly rare on their world. They power an interstellar spacecraft with the few pounds of it they can manage to scrape together to go out looking for more, which they find ... in the form of a spaceship from Earth. The Nevians and "Tellurians" get off to a rather rocky start because of this, but eventually become friends.
When Star Trek: Voyager started they were in an area of space where water was the go-to commodity. Our heroes can make all they want (within reason) and find themselves a common target because of it.
There's an episode of Only Fools And Horses which uses this as the punchline to a Violin Scam: the boys raise a huge amount of money to buy a rare Hindu statue from one man, intending to sell it to another. Both men then disappear leaving them with the statue ... and then they find an identical statue in an Indian restaurant, and the manager tells them he got it for a couple of pounds at a Portobello Road stall filled with them.
Restaurant Manager: It's amazing the bargains you can find if you shop around.
In the Star Trek: The Original Series episode "Elaan of Troyius", the ship's dilithium crystals crack in the middle of a battle. Unfortunately, there are none left... until they realise that Elaan's necklace has a bunch of them. She surrenders it gladly, bemused that they would want what to her planet are Worthless Yellow Rocks.
In Paper Mario: The Thousand-Year Door, after returning to a mafia Don who was part fo a previous quest, you find him deathly ill and his bodyguards promise you the tickets for a trip on the Excess Express you need if you find his daughter and son-in-law (who he exiled at the end of said quest). On recovering when they return to his side he's furious... until you state what you were promised; he bursts into laughter and asks how many you want.
A variant in Escape from Monkey Island. There are two different puzzles you can solve to obtain the Mega Monkey Meal Coupon; once you redeem the coupon, you will have the opportunity to obtain a part of the MacGuffin you're trying to assemble, but it's possible to miss your chance. Rather than make the game Unwinnable, once you have solved either of the necessary puzzles, an entire barrel of Mega Monkey Meal Coupons will appear, ripe for the taking.
In Megaman Battle Network 2, one NPC asks to be payed in "Guard * chips". Talk to another NPC, and she'll give you 30 of them.
"They are sentient, you know. We just have a lot of them."
In Fallout 2, Vault City has an entire apartment full of water chips, the object that half the first game is spent trying to obtainnote Even worse, they were intended for Vault 13 - the vault in the original game that needs it - but were sent to Vault 8 (Vault City) due to a shipping error.
Annihilation amulets in Secret of Evermore: 10,000 gems to get one if you don't want to cross the desert on foot, and once on the other side you can easily buy a couple others in the marketplace.
Discussed by Sam in one strip of Freefall. Since Jean is in the process of being terraformed, wood and other organics are extremely rare and valuable, while on his planet, the landfills are full of cheap wood items. On the other hand, diamonds are so cheap on Jean that they're thrown away, while a diamond grill he found in the trash would be worth a king's ransom on his planet.
In the Futurama episode Time Keeps On Slippin', a character mentions that they'll need "some sort of doomsday device" to solve the problem of the week. Quoth Prof. Farnsworth;
Farnsworth:Doomsday device? Well now the ball's in Farnsworth's court!
[The professor presses a button on a remote, causing several different stereotypically "mad scientist" style machines to come up through a trap door]
Farnsworth: I suppose I could part with one and still be feared.
In Aladdin: The Series, Al and Iago go on a quest for the legendary Orb of Macana, the only thing that could cure a sick Genie. They have to overcome three arduous trials, and after completing each one, the guardian of the Orb goes to a chamber and retrieves it... from among thousands of identical orbs. The guardian complains that at this rate he'll never be able to get rid of them all.
Porky in Wackyland is all about Porky Pig trying to capture the last of the Do-Do birds. After he does, the Do-Do replies "Yes, I'm really the last of the Do-Dos. Ain't I, fellas?" Suddenly, dozens of Do-Dos appear out of nowhere to answer him.
In one of the Three Bears Looney Tunes ("The Bee-deviled Bruin"), Papa Bear nearly gets himself killed trying to get honey from a hive in a tree outside his home. Eventually, he gives up and asks for a bottle of ketchup. Mama Bear goes to get it... from a cupboard filled to the brim with jars of honey.
This will happen quite a lot with food, among other things. In Spain, especially in Galicia (the northwest), the vieira is a rare, expensive mollusc that is frequently carefully prepared and served, in its shell, one to a person as a tapa. In the United States, especially in the Northeast (more or less due west from Galicia), scallops are frequently served in large handfuls with bread crumbs and butter, broiled in a disposable pie plate, and are generally the whole meal along with a beer and some bread.