I have put up a small lot of JMF items in the annual Con or Bust auction.
This includes some things people here probably already have (The Dragon Waiting, Liavek: Festival Week), but also includes some old Asimov's issues with material that is not included in either Heat of Fusion or From the End of the Twentieth Century:
- "Hot Pursuit," a story which has not ever been reprinted.
- "Slowly By, Lorena," one of the four Alternities Inc. stories. It has only been reprinted in The Fantastic Civil War (a Baen alternate history anthology from 1991).
- "What's Wrong with This Picture?," a collaboration with Barry Longyear and George Scithers which was only reprinted in Tales from the Spaceport Bar (a theme anthology from 1987). (The story is admittedly an extended joke.)
- "On Playing Roles: A Second Look," one of a (short) series of role-playing game reviews. It's fairly lengthy and (I think) interesting despite its age, assuming you have any interest in RPGs. This has never been reprinted.
- "On Opening Night at the Universe," which is about the premiere of Carl Sagan's Cosmos TV show at the National Academy of Sciences. This too has never been reprinted.
There are of course many other interesting things in the Con or Bust auction.