Shirty Something
We had some printed up by Shirty Something, great people to deal with. We had a simple logo screen printed on M to XL shirts, which worked out at around £6 per shirt IIRC.
One thing about the above design, is that it may be a bit too complex for a traditional screen print, but there is a "direct to garment" option, which is a bit more expensive.
Make sure that you're asking for screen printing, as a lot of places will offer vinyl printing, which is a lot less forgiving, less flexible and will crack.