If in fact that is the proper manual for the spreader you have, does that manual have a parts section with part numbers..?? I have found in the past it's easiest to find hard to find parts using the OEM part number rather than just a generic description of a certain model. Using the OEM part number you can do a search using Google, or ebay. I've found many hard to find parts that way.
A salvage yard for farm machinery would be your best bet. If you go to TractorHouse
TractorHouse.com | New & Used Farm Equipment For Sale at the top right of the page, you'll see post a "want to buy ad". Many smaller salvage yards are connected to this site. You'll have to register on that site giving contact info, to weed out spammers. Enter your make, model, part number, and a general description of said part you're looking for.
It's a great tool for finding hard to find parts. I've found more than several parts that way.
