Depending on how many you want done you can do this yourself
with an ink jet printer, print the image onto the transfer paper and
iron on to the t-shirt, I got some in Easons but I am sure it is available
in most print/office supplies. Of course if you want hundreds I wouldnt
recommend it :)
I bought some from here some time back, I haven’t had a chance to try it yet but his prices are good. The one I got was for printing on dark materials. I must get it out of the packet and give it a try.