Back in a previous life, I used to work at a silkscreen T-shirt factory. There was a decent one-time set-up charge, to cut the screens-- but once this was done, the per-shirt charge was pretty reasonable (pricing depended on the quality of the shirt and the number of colors), and the screens were kept on file in case there were repeat orders.

Any vendor should be able to send you samples, so you can check out the quality of the printing.

I'd suggest you try to find someone reasonably local, so you can pick up the shirts yourself and avoid the shipping costs, and can check out the quality in person. Plus, many shops have designers in-house, who can help you if you don't already have a finished shirt design.