See Sheldon Brown:
http://www.sheldonbrown.com/english-3.html#tire

If you look for EA3, or 590, or 650A, or 26x1 3/8", you can find a surprising number of tires. Some wheel chairs also use this tire size. Someone on this board put together a list of tires he's found in bike shops, hardware stores, big box stores, internet, and discount stores, and it was quite long.