I'd be interested in hearing what you find. All I've found are the highway tires. The best price I found was at a local tire store (and they said they'd have them the next day) for about $150 for tire, tube and flap plus mounting and balancing. The mounting and balancing has to be done at a truck tire place due to the size and multi-part rim. I've also done some looking for the 19.5 inch rims but haven't had any luck.
Be careful of using any smaller diameter rims, they may not fit over the rear brake drum. I was able to put 16 inch rims on front but the width of the rim allowed the tire to rub on the tie rod ends and cut the tire. The 16s didn't even come close to fitting over the rear drums. Anyway, I thought I'd pass along what I've found so far.
