Did you look on the DISigners board here? Because they are willing to design something for you if you don't see something on there you already like. Just be sure to read all the "rules for requesting" type stickys at the top of their board. They'll design the image for you then you just print it out on iron on transfer paper and iron it onto blank shirts. Most of the people who make a lot of iron on shirts (like me) order our iron on paper from a fellow DIS member who has a website. Google "Amy Mickey iron on" and you will find her site. She only sells iron on paper for dark colored shirts, but that's what we use for all colored shirts because it gives the most colorful results. Just remember that any blank space will be white so you have to choose a design that's easy to trim around. Check out the Creative DISigns forum for more details, you'll find everything you need to know about making shirts there.
ETA: I order my blank shirts from Jiffy Shirts. Good prices, large selection, and fast shipping.