Dan, welcome to the forum!
Glad you found us!
You can buy your tires from any tractor dealer. If they don't have your size in stock they should be able to order them for you.
You can also find them on the net and order there. You would need to find someone local to mount them for you. Be careful, the rear tires are sometimes loaded with liquid to put weight on the back of the tractor. Messy if you don't know it is there. Put new valve stems in too.
Good luck,
Jim