Check if you have any local/independent stores around town. A local chain in my city sells DC, PS1, SNES, NES, GEN and other classic games. As for national chains though, nope. Other places to look at are flea markets and yard sales.
No at GS/EB (and all the other companies they own). You can still find DC games at GameCrazy (if they haven't been shipped to a diff store) but it will be pretty slim pickens left, like just about all sports titles, discs only, and damaged stuff.
Don't know if you have one, but try Gamers if there's one near you: [SIZE=-1]http://www.gogamers.com/ . Might not be much online, but in the store, they have a lot to choose from.
CD Warehouse is also a great place. I found all sorts of games, although you might wind up with more Sports games...
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I recently got out my Dreamcast to play some old fighters after a thread about Marvel Vs Capcom 2 made me want to play it again. My sites that I used to use to get games right when the dreamcast went to 50 dollars are all either no more, or simply don't stock DC games anymore. I do see on average at least 20 games for the DC at the two used cd/game stores I frequent. Mostly sports titles but that tends to be the case with most old consoles. If you are willing to risk it, I do see dreamcast games still for sale on amazon market place, but I rarely see any in stores anymore.