You can buy tires almost any where but the rim is different. OEM will be the best option unless you find them used.
Rear Tires and wheels:
Gravely 07100217 Tire, 21 x 7.00-10
Front Tires and Wheels:
Gravely 07100219 Tire, 13 x 5.00-6
Now about the PTO, that bugger is costly so we should make sure it is broken and not a safety switch malfunction.
Gravely 03789900 Clutch, Ogura Price $232.89