Assuming you are asking about the INSIDE wheel well liner and not the outside trim.
Your most likely going to have to fabricate one yourself or find a sheet metal shop fabricator which can make it for you.. Wheel well liners are not a generic item, tend to be custom built for your trailer.
The outside trim called wheel skirts is also a custom made item for your model only, there are aftermarket makers carrying some different sizes fitting specific brands and models but they tend to be priced is out of this world..
HERE is one example of a place selling aftermarket wheel skirts..
HERE is a general Internet search for trailer wheel skirts..