http://shop.fsf.org/category/gnu-gear/
There are some GNU shirts and other stuff.I once had an idea about a shirt design, it turned out very basic, but I'll just put it in here as a proof of concept. If you are good with gimp you'll surely be able to do something out of it, that is worthy of being put on a shirt! I still have the original .xcf file, if you want it.
