You should be able to get trim moulding at any real RV store. I've seen various types on ebay.
Another place is at the link: https://www.all-rite.com/rv-parts-accessories.
Another and one of my favorites is Bontrager's Surplus. We stop there when we are out traveling and I prefer to look through all the stuff for those items you just have to have, but they sell on line I think. I've found they have reasonable on most things.
I'm editing this as I just looked at the all-rite site, and I have to say, they have outrageous prices.
