You can go to almost any office supply store and buy t-shirt transfers that you can print at home. Just print the design in reverse and iron it on accoring to the instructions. They cost about $1 per each 8.5" x 11" sheet.
UPDATE: The t-shirt transfers are just sheets of paper. You print them, cut the design out with scissors, then iron them onto the shirt. They last as long as the shirt does. The only cartridge you would need is an ink cartridge for your printer. That would probably be cheaper than paying somebody to make one for you.
Hey, we do t-shirt printing.....just attach what you'd like on the t-shirt in an image file, how many t-shirts you'd like and send us an e-mail to firstname.lastname@example.org
Our website's below.