T-shirts (and other items) are on sale at Giant Robot. Here's the direct links to the food monster, cat & elephant, bored monster, and skull Transformer logo.
Relatedly, you can see the top entries in the Giant Robot/Shin Tanaka papercraft design contest here. I actually think this one by Ricardo Munoz is my favorite:
*Previously: Optimus Picante t-shirt.
*Buy Transformers at eBay.