Found this seller for the best price. I'd contact them to see if they can get it.
https://www.smallenginesprodealer.co...onan-166-0821/
Can you explain why you think you need this part? What are the current symptoms? It's not that common for it to go bad.
For what it's worth, I keep a complete Onan Marquis 7000 as a spare. I picked one up, still mounted to the original shipping crate, for $1100 a few years ago. It was used as a backup generator for a work shed. The owner converted it over to 240V single phase, but I converted it back to 120V.