The 3rd Rail Store is a place for artists to showcase and sell their designs. They’ve got several designs I like, but this one, Smoking Chimps, is probably my favorite. The artist for this particular design is Joanne Pennington.
You can buy the shirt, and check out the rest of the designs, at the 3rd Rail Store. They’re located in the UK, so all the prices are in GBP. Smoking Chimps costs 20 pounds (which is about $30).