check your phone book for a local screen printing shop. If you just want a plain white shirt with one color letting it will be just as cheap for an order that size to have it locally done. If you have more than one color or want a diff shirt than white, the internet will be a little cheaper, but then you have to hassle with shipping and stuff.
All the sites are pretty much the same, its hard to [censored] up a t-shirt, so just look for one that has a decent web page.
google search result