The factory, unless someone knows who they buy from that you could by direct.....
As long as they are the same size and capacity of the current rims/tires, I don't see why it couldn't work.
Computers manufactured by companies such as IBM, Compaq and millions of others are by far the most popular with about 70 million machines in use worldwide. Macintosh fans note that cockroaches are far more numerous than humans and that numbers alone do not denote a higher life form. -NY Times 11/91