Walt,
The first clutch adjustment should have been done when the tractor was brand new. They are rarely set correctly at the factory.
If you need a new clutch disc(s) there are no OEM heavy-duty models made as far as I know. However, you might be able to have your old clutch discs re-lined by a clutch rebuilder with heavier-duty material. You can't increase the thickness of the clutch facing though, or you will throw off all the settings and may result in a non-functional clutch.