We get a number of requests of places to get buy or license t-shirt designs. Below are a few resources to get you started:
Visit Designious for a large selection of t-shirt designs available for purchase for under $10.00. You can also find more resources for designing t-shirts including vector packs, photoshop brushes, and fonts.
The T-Shirt Factory has over 1000 print ready t-shirt designs to choose from. Prices are reasonable at only about $10 for a royalty free license.