So I found this site called GameQuestdirect.com that seems to cater to the less heavily printed/more obscure game titles. They seem to have fairly decent prices too, but they don't know how to organize games by genres (Tetris in platformers? Kingdom Under Fire in shooters but KOF 2 in action?)
http://gamequestdirect.com/index.html
psssst, they have Rez in stock!