Local cons on university campuses. Seriously. A largish university's rpg club hosts a convention and invites local FLGS'es for a small dealer room. These vendors will bring new stuff but try to sell off backstock of old stuff at good prices.
At my local con, on Sunday afternoon as the dealer's room was packing up, I happened on a vendor who had a number of "vintage" books still out. One was a box set that I asked to look in. It was the Basic (Red Box) box with many items in it: the Red Basic Set books, the Blue Expert Set book and module X1, the White Companion Set book, and a copy of Keep on the Borderlands, all just shoved into this box, all in not bad condition.
I asked him how much for individual items. The dealer shrugged and said "give you the whole box for $15." I asked if he was sure and he said he'd rather I have it than have to haul it back to the store and let it sit. True collectors want mint condition; I like to collect because I like reading the old stuff and may want to play someday.
Anyway, local cons is my advice.