I'm guessing you mean the white plastic delrin washer that fits over the rudder stock - roughly 4" OD, 3" ID and 1/4" or so thick. If so, the rudder was almost certainly made by Foss Foam and your can find them at
www.newrudders.com. Or if you have wood working skills and tools, you can buy delrin sheet stock from McMaster Carr and make one.