I had no idea wheelchairs were 650A. Economically, it would make more sense to use the 650A because of the bigger base of potential customers. I guess it's a lot more romantic to use tires from a bygone French stantdard than a wheelchair.
Well 650 is 26 x 1 3/8" which aren't the widest tires. I had a 3 speed for a while with that tire size and it was OK, but I can see where people would want the wider tires. Isn't the point of 650b conversions mainly to help re-purpose old road frames to be better commuters with wider tires and fenders?
Quick search shows Schawlbe, Panracer and Michelin all make 650a. Seems like there is some good quality out there.