@weatherproof, Oops! I totallty missed the word "glass" in your question. The Oleson and Maxwell screens are, of course plastic, so I assumed you wanted something in that category. All of the new, third-party screens I am aware of are made of plastic, so my reply was off topic. Sorry.
Maybe less so with Mamiyas, but a lot of people are switching to brighter Fresnel plastic viewing screens for their Rolleis, which means there should be a some used glass screens out there. On the other hand, I am not going to part with the glass screen that came out of my Rolleicord until I'm sure I like the new Oleson, so maybe people are hanging on to the glass ones?
I've only ever used two TLRs in my life, a Mamiya C220 and my Rolleicord V. For me, the Mamiya's factory glass viewfinder screen was acceptable, but the factory Rolleicord V screen was not, because the edges and corners are so dark. Perhaps someone with more experience can say for sure, but I'm guessing it might be hard to find another glass screen which is any better than the one that came in your Mamiya?