Well, from a bicycle shop (the late BikeSpring) of course. Alright, they don't really carry 339x2.5 tires, but then again the rim we're working with here is itself a tube, so it's not like a normal clincher tire will work. Or will it? Why yes, with a little surgery. The solution is to take normal mountain bike tires, cut them open, cut the bead off, and straighten them out so they have a larger diameter curve. The tire's response is sometimes to twist itself into a spiral, but with a little force it'll stay reasonably straight.
The idea was to glue on the tires; the first pass was done with Goop adhesives, but it's a bit hard to work with in volume so we stopped and used zip-ties... which don't prevent twisting much at all. More recently, we used neoprene contact cement, easy to work with as you can see in this short video of Jon and I gluing on the tires for one wheel, 4.5 tires per tire.
