After 10 minutes of research, there are several websites like vistaprint and cafepress that will let you upload and share your designs and buy singles of shirts. It may be worth starting another thread where people just post designs they've made and we can just buy and/or upvote the ones we like.
Unfortunately, I can't contribute to this myself other than the buying/upvoting part, because my designs WILL look awful. There are templates on those websites as well for people like me but I can't even stand to look at the ones I've made even with the templates.
I'd make the thread myself but this isn't really my baby, I'm just dumping what I know here and hoping to guide the discussion toward someone out there making a shirt design that I'd actually buy.
Oh yeah, and now my facebook is cluttered with ads for vistaprint. Isn't that great?