ebay, amazon and play.com but occasionally i'll find a bargain in my local big supermarket (Tesco or ASDA)
i used to think less of used games, but i stopped caring when i realised that it didnt make any difference whether or not that disc had been used before, once its been in your machine just one time its not "new" anymore anyway.
all i bother about when im buying games on ebay is whether they have a manual and the club nintendo points (wii/gc games obv) because i can trade them for wii points XD