I just got a thrust washer from an OS.28, I think it was. Current production product and it fits. Thrust washer from .25F will fit also.. i'll have to see if I saved the header card from the package.
Type at you later,
Dan McEntee
Edit to Add: I found the header card. OS Part#22808000 Drive Washer for 21RF, SE,25FX,32SX This is the washer that the prop butts up against. The thin washer behind it is called the thrust washer. It should be available for the above engines also. I ordered these through the local hobby shop from Great Planes/Tower Hobbies.