I got my bonded washers from Tommy at Affordable Tractor Sales in Texas. He knows which ones you need and should have them in stock. As I recall, the ones on my ZL-30 FEL are 18 mm, but I could be wrong.
Well I should have listened. I read somewhere that I was supposed to measure the bonded seal from the ID of the rubber part. So I ordered 10 14mm parts. to be safe I ordered 12mm and 16mm parts also. the 16mm parts were still too small. Now I am re-ordering 18mm washers. Its funny I was in a napa store in Hillsboro OR last week asking about these. They had no clue what I was talking about. They were convincing me I had some very specialized piece of machinery. I didn't elaborate and let them continue to think I had a very high end machine.