Half.com or eBay? Don't bother with Game Stop -- their used games cost almost as much as new ones! Sometimes Toys R Us has good sales on DS games, too.
I've gotten some Club Penguin and My Sims games from Amazon for less than $10. I always look for their sale, cheap, markdown games. I do not like paying full price for a ds game. On occasion, I've seen games for $10-$15 at Wal Mart and Target (clearance).