I was at the John Deere place picking up blades for my mower and they had a box on the counter. Plain black knob with chrome clamp.
http://www.amazon.com/s?ie=UTF8&rh=n%3A15684181%2Ck%3Asteering%20wheel%20knob&page=1 Seems like there is very little you CAN'T buy from Amazon.