The big one to check is Noble Knight Games, which has a ton of OOP products for the going market rate or less (you can occassionally find really good bargains). I order almost all of my OOP games (and I have a lot of them) from Noble Knight.
NKG carries a ton of ultra-rare stuff, as well. Frex, I purchased the only physical copies of The All Australian Roleplaying Game (Hunter Planet) and Zombi that I've ever laid eyes on from NKG. I also snagged some first printing TWERPS supplements and GURPS Man to Man from them.