I've had hundreds of these things go to print through commission or design contests. Here's a sample of my favorites, many of which are available at reputable shirtmongers like Shirt.woot, Threadless, Design By Humans, and my own little shop. You can also find a lot of it for sale by sneaky people who pass it off as their own. Dang!
Occasionally I'll take on client work, especially if it means free skate boards and power tools. There's some other stuff here, too.