Online T-Shirt stores tend to be a dime a dozen these days. Hell, cafepress lets anyone at all sell their ideas and they take care of all the leg work even. All you do is upload the image.
And then there are places like Busted Tee's and T-Shirt Hell, where comical creations take shape and where insults and politics collide. These are certainly worthy retailers, although beware of T-Shirt Hell. I'm not really certain where you could wear most of their designs.
Still, of the vast number of iron on artists the hands down winner is Palmer Cash. They've been in operation for years and from the jump they were ahead of the rest. The really remarkable thing about PC's style is the way it reminds you of when T-Shirts were cool. In the late 70's and early 80's when T-Shirts weren't about displaying your favorite athletic company or rapper turned designer. The Palmer Cash designs are completely original, unique, amusing, enchanting, reminiscent... Seriously. I could go on all day. It's the only online store that has my real email address b/c I actually look forward to seeing the new additions.
Shirts range from $15 - $25 and shipping is a flat $4.50. So pile up on them.