I have quite a bit from Woot and Threadless. But I've been shopping http://www.karmaloop.com/ a bit more recently.
Other sites I tend to hit are
www.glamourkills.com
www.bleedingstarclothing.com/
www.tanktheory.com
www.boomslank.com
www.designbyhumans.com
www.goodietwosleeves.com/
http://www.sciencefriction.bigcartel.com/
Some of these do tend to run expensive, I'd put johnny cupcakes here too, but I don't actually own anything from them. A lot of these places are run by like hipsters, so you do have to deal with like don't eat meat bumper stickers being sent with your shirts >.<
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Chrono1219