cafepress.com has all kinds of really cool T shirts. It is fun to browse and see all that they have!!
Look at These

cool

Insanely Great, which also happens to make the fluther tshirt, has a lot of great mac designs.

On eBay, if you search ”apple store shirt”, you’ll find a ton of shirts that are given out to employees of the apple store. Pretty unique because not a lot of people will have them.

Personally i’d love to see this one out and about… There’s a couple of cool ones here.
But so far the best i’ve ever seen were on Insanely Great just like the monkey said.

I remember this corny one from when the iMac had just come out.