I get almost all my t-shirts from Threadless these days. I can't really beat the price or quality, so don't shop around much and just stick with the Threadless goodness.
Which is certainly not to say I wouldn't shop elsewhere. I'm always on the look out for the next not-so-big-because-big-would-be-too-popular thing. This one's a neat alternative: Woot Shirts.
Threadless sells their shirt until their batch sells out. Woot sells each shirt for just one day. Elitister! Threadless costs 15-20 dollars, Woot costs 10. Elitister! Well not really, less elitist really. Thriftier!
Anyway, Woot Shirts.