They're not making them for motorcycles anymore, but I originally purchased mine from Donahue Harley-Davidson. Brett Donahue, the owner, is an Iron Butt Rally veteran. I'm not sure if they still have any, but a quick call to his parts department could tell you. LINK
I should note that a few months ago I had a sensor explode apart in my front tire. Tire was fine... But, I needed a new sensor so I called up Donahue H-D and they had complete units sitting there. So I ended up picking up a complete new setup. Front sensor replaced, and now I have a spare sensor and a spare display should anything else break.