Ebay! Just browse thorugh the games for sale and check if they're any good on MobyGames.com. Then buy them for € 2 each :P
Not very heoric I must admit, but a genuine treasure trove nonetheless
k:
I did get Duke Nukem "Manhattan Project" for € 2,50 from some bargain bin for € 2,50 but this was also a gamble since I didn't knwo the game. Turned out pretty well :P