I know it's not a 122, but we recently upped the size on our Original to 26x12x12s through Miller Tire. We bought their rim tubes and centers and then welded them to fit our tractor. Price can be well below the original $500 you mention and, you can be sure they fit without spacers. You can check out their website to see if they're attending a show near you and they can deliver them to the show so you save on shipping.
Here's the finished product.
For reference, the right tire in the rim shot is a 23x10.5x12, bare rim is their 10-inch tube being fitted to fender.
And no, we weren't pulling on the surface of Mars. For some reason, my 'puter likes to change the colors around a little bit when I save photos.
