Internal rubber exhaust flappers
 

I'm looking for replacement rubber flappers. These are a 4" round disk of steel impregnated rubber that is held in place with two screws at the top of the exhaust tip. (There is no shaft, the rubber flexes to allow the exhaust to pass). At some point I'm going to replace the tips to a mechanical flapper but right now I'd like to splash the boat and try out the new motors...

Any ideas on where to get these?