Cafepress basically, sends you a layout, and then you make your shirt or whatever. I order from my favorite artist back then. One being "Michael Poe" AKA "Errant Story" AKA "Exploitation now". Then of course, their were some other artists as well.
If you want to print specific shirts, you could go to a retailer, nearby, but I suggest making a shirt from scratch, and just printing the material. Of course i am from the big city, where I could find a shirt printer, not too far from me. Maybe a day walk or so. And they will do the thread work, and make it into an shirt, on the spot. However it gets pricey.
Cafepress is basically what deviantart is trying to do. Print your stuff, using their services, charging a small fee, to the artist. Their are tons of people out their who does that. My sister also was trying to do that, business for awhile, since she has the machine, for it.
Cafepress had a lawsuit, because it would advertise the materials on the front page without paying any royalties to the artists. If you think you will like it, then go for it. Otherwise, I am too big for that nowadays, knowing how to do what they are doing.