I believe in the "If not you then who?" method of finding what you need. I'd contact well know transmission oriented people such as David Kee and others and ask them. I know Kee does toploaders, but he's been around forever. If he can't get you what you need he likely knows who can. Also a Google search turns up a place called PitStop USA as a Richmond parts suppplier. Did you try them? Is this the part you're looking for?
https://pitstopusa.com/i-23925204?q=richmond