I have a buddy that does shirts when i want some done. He doesn't really do shirts too often though, so he would have to order the shirts themselves. The set up he has requires polyester shirts because it evaporates the ink to put it into them. If the design and everything is kept simple he'll probably do them for what it costs him pretty much. The only color shirts he cannot do are really dark colors.
Just throwing that out there if people are interested. There are other places around london here that do them too and can do whatever material you want the shirt to be. The lettering is basically just iron on simple font stuff though. I can usually get a pretty good deal from them too