On Amazon (Click here) or any RV dealership parts department. If the trim on your shower is plastic, there's probably not much you can do with it, except re-paint it with a paint designed for plastics.
I've repainted a LOT of plastic trim pieces on all my campers. Old Fashioned Rust-o-lium works best and lasts forever. I started using Rust-o-lium Professional. It's even better.
Note that Rust-o-lium Professional is an oil based paint. It is not water based. So clean up requires either mineral spirits, kerosene, or gasoline to clean up the brushes if you use them - not water.