it is fairly slippery and tough (generic name is Acetal, Delrin is a trade name). This is just the washer taking the thrust of the rudder weight, right? You can buy Delrin AF sheet (AF is the teflon bearing version) from McMaster is various thicknesses up to pretty thick, which you might be able to cut out yourself. It machines pretty nicely with woodworking type tools (hole saws, band saws, etc). PEEK and Torlon are much stronger, stiffer, and considerably more expensive. Not needed or even desirable in this application. UHMWPE would also do the trick. I made one from that for my last boat and its still on there. The PE is about as slippery, more "rubbery" to machine, and a little less brittle compared to Delrin, but not as strong or stiff.