I buy a lot of t-shirts from a lot of different online sources.
Some of the tshirts are probably a small bit offensive, so you have been warned 🙂
T-Shirt Hell - some of these designs are very clever, some are simply offensive. Some of the cultural references don't make any sense to me, but probably make perfect sense to Americans.
Despair - I simply love their stuff. Signup to their newsletter to find out about the limited editions
WB Shop - some very nice film and TV stuff. NB: some will not ship outside the US, so get yourself a myus.com account. Using myus.com also solves the problem with WB not putting the value of the items on the package
Shot Dead In The Head - nice mixture of popular culture designs.
Think Geek - they have a very good selection of shirts. Some are geeky, others are popular culture. Worth bookmarking!
Busted Tees - nice selection of movie and tv tshirts. Some gems
Last Exit To Nowhere - very very cool movie tshirts. If you aren't a movie buff these probably won't mean a lot to you. If you are you'll love it!
I've also picked up a few from Zazzle, though they're pretty much hit and miss ..