Some old 80's Schwinn MTN bikes also used 650b wheels (Mirada I think was one). 26" to 650b can be done sometimes. I tried a vintage 26" mtn frameset I had sitting around and put on some 650b wheels (from a Mirada) and with some V-Brakes and the pads almost all the way to the top worked perfectly. The Mirada wheels I used as my test basis for my PX10 conversion. I was able to test fit the wheels and measure the brake reach I needed before ever spending money. I even upgraded the rear Mirada wheel to Q/R, but wheel wasnt very round anymore so I ordered the parts to build up new wheels.

As for mtn 650b tires there are the two tires from Pacenti, Kenda Nevegal, Interloc Racing Fire Cross, WTB Wolverine, and maybe one or two others I am forgetting. I found this list of 650b rims that lists everything 650b rim wise. The 29er movement basically started in the late 90's when after some industry pressure WTB started making the Nanoraptor. Gary Fisher was the first big brand on board. I the beginning you were riding one of only a few 29er tires aboard a Gary Fisher or a handmade custom. For you fix of offroading 650b I suggest visiting http://650bpalace.blogspot.com/