We've always had good experiences with
http://www.greekshop1.com/. I know the website looks atrocious, but it's a good company. They've always done great work for us. They're always very prompt with our orders, and when they aren't, they call to let us know.
Also, a great site for designing your own t-shirts (like for rush or mixers or whatever) is
http://www.customink.com/. They have a thing where you can plan out your design and everything, and it's very handy. They're always very prompt and courteous; I don't think it's ever taken them more than two weeks for them to finish and order and get it to us.