I bought mine at Les Schwab tire stores and had them mounted there. Be sure they clean the rim and put on a new valve stem as I experienced some air leakage with the original installations that had to be subsequently fixed by another of their shops (no charge).
I bought my new tires at my local favorite independent tire dealer. Gave me an option of standard 4 ply or to 6 ply, only a couple of dollars per tire more. They work well. No need to radial tires, waste of money even if you could find them in 12".