You might try eBay.  Be sure to use original Hopper part number from your op manual, or call dealer to get part #.  Lots of vendors on eBay include original part # in their heading.  BE SURE TO POLISH IT AND TO APPLY NEVER SEIZE OR WHITE LITHIUM GREASE TO IT BEFORE REASSEMLY!!!  Can save a ton of labor if later you need to R&R the PTO clutch.