As I mentioned, Stens has very good quality parts. Rotary does too. I was set up with both, as sometimes one would have what the other didn't have.
Onan parts are always going to be a problem. I never found a good aftermarket source for Onan parts, but rarely saw an Onan come into the shop anyway. I just got the few parts I needed from an Onan dealer, who specialized in Onan RV gensets. He could usually find what I needed.