The tire size for the minipro is 70/65-6.5 and I found it on
http://www.aliexpress.com/ but it looks like the minimum order is 2 tires. I have never ordered from them before but that's the only place I have seen with that size listed.
I'm sure in the coming months spare parts will become available since all things mechanical break.
That sounds pretty stupid if you have to return it to Segway for them to replace the tires!