Of the 1974-76 NBC run, two episodes exist on video (a master copy of June 11, 1975 and an Andy Warhol-taped copy of July 4, 1975, both held by the Paley Center for Media) and three more on audio (the July 1, 1974 premiere; October 15, 1974; and July 15, 1975 are held by Archival Television Audio, Inc.). The 1983 Daytime Emmy Awards showed a 1976 clip, known to be from the last seven weeks.
The 1975-76 syndicated run is unknown.
Twelve episodes exist of the 1978-80 revival:
1978: February 14 (Pilot #2, held by UCLA) and May 19.
1979: March 2, March 27, December 4, and December 31.
1980: February 28, March 20, March 25, May 7, June 9 (on audio), and the June 20 finale.
The Martindale era was rerun on USA from 1988-91. The 1986 pilot also circulates.
Screwed by the Network: NBC moved the show on January 19, 1976 from Noon to 10:30, putting it up against the second half of CBS' The Price Is Right. Right before it was canned six months later, desperate times called for desperate measures (see the YMMV tab for that).