I use genuine Sanitaire commercial belts because Vibra Groomer II brush rolls can bow if you store your vac with the wrong belt and I don’t want to ruin mine. To me, genuine seemed the safest bet.
That said, I suppose you could try a “Perfect” brand belt or perhaps even a Koblenz one. I recall a member on this site saying that she was able to get a lot of miles out of a clear Koblenz belt in her Sanitaire.