Here is a list of tshirt sellers that I rate. There are a million more, many of which sell crap - but if there’s a quality site out there I have missed (whether it is yours or not), let me know either below or via email: 
- Bad Designer Threads - posts (none yet) - review (coming soon)
- DesignByHumans - posts - review (coming soon)
- Gama Go - posts (none yet) - review (coming soon)
- Go Ape - posts - review (coming soon)
- Imaginary Foundation - posts (none yet) - review (coming soon)
- Johnny Cupcakes - posts - review (coming soon)
- Oddica - posts - review (coming soon)
- Old Crappy Tee - posts (none yet) - review (coming soon)
- OneHorseShy - posts - review (coming soon)
- Owl Movement - posts (none yet) - review (coming soon)
- P is for Panda - posts (none yet) - review (coming soon)
- Snorgtees - posts - review (coming soon)
- T-shirt hell - posts - review (coming soon)
- Ten Bills - posts - review (coming soon)
- Thinkgeek - posts - review (coming soon)
- Threadless - posts - review (coming soon)
- Without Pockets - posts (none yet) - review (coming soon)