I have 2 t-shirts, featuring a very patriotic Harley as the main topic of the artwork, with Hidden Mickeys throughout the bike, and "Walt Disney World" underneath. However, Mickey himself is not riding it.
I bought one of them in a gift shop on Main Street USA, and the other in the Wonders of Life pavillion before it shut down.
I have also seen them for sale at a gift stand on New York Street in Disney/MGM Studios.