I have bought shirts from most of the usual suspects, and the DBH shirts seem to be the highest quality. I did rip a few stitches on one of the collars on one of them whilst taking it off during an exercise session, but that was more my ":hot:OMG I am going to die, get it off of me!:hot:" fault...
If you haven't checked out Design By Humans, give them a shot: http://www.designbyhumans.com/
They don't do mashups very frequently at all, and they have a great selection of cool shirts. Some of the shirts are quite odd, though, so consider yourself warned!
I am hooked on the various shirt...