Oh man.
Garage sales, estate sales, flea markets and antique stores are my JAM.
I have been very lucky this year. I found some first editions ( Eaters of the Dead by Michael Crichton, Christine by Stephen Hack King, and Smith of Wootton Major by JRR Tolkien), but my best find was about a month ago.
I bought about 20-25 original theatrical one sheet posters from a guy who used to own a theater. A couple were folded, but all had been displayed framed so no pin holes. Great selection including Rambo 2, Rocky IV, Ghostbusters, Back to the Future, Highlander, Dune, Pee Wee's Big Adventure, Die Hard, Commando, Young Guns, Rain Man, Terminator, Silverado, Enemy Mine, Star Trek 3, Return of the Jedi ( 1984 re-release)... it was probably my best find ever. Paid $40 for the lot!