If you live out in the country many local tire shops carry and install tractor tires. Some of them even come out to your place and do it there. Not many stiill do and they're getting fewer and fewer.
When it comes to installing new tires I go to the local Cooper retailer who does a lot of ag work. He also sells about every brand and make of tire you could want. I replaced the original 6-ply Titans that came on mine with 10-ply Supermax. I don't even know who makes them or where but it was the dealers recommendation and he hasn't let me down yet.