Drain pipe collar

Rubber sleeve with two hose clamps? Any big box store will have slip collars. Ask for a fernco collar.

I would make one out of sheet metal it will last longer than the plastic ones.

Perhaps try Factory RV Surplus. I have found some one-off odd stuff there.

I'm not sure what piece you are talking about picture would help. In any case, I’ve found a lot of parts on Amazon, eBay, Tweetys, and eTrailer.

Amazon type in 3" PVC trim ring its like 8 bucks

Thanks for all the answers, had already checked them out. Will make my own when I get home for Christmas. Ron