If anything, aren't 650b road wheels, and particularly rim brake, HARDER to find than 700x23 tires? I've considered going down this path on a couple of my bikes, by using long-reach calipers so as to get a fatter tire onto an aging road frame, but whenever I've priced it out or gone googling for the needed bits and bobs I always came back to "well I guess I'll just squeeze some 28/30c into the space I do have and be happy."