this really depends on the age of your trailer.
If it is recent, the manufacturer or dealer should have what you need If older, especially if you could call it "vintage" you may have to go to the classic suppliers like Caro.
Hold on to your wallet.
I had a corner of the rock guard blow off, and peel away while crossing the Great Salt Lake. After calling every RV salvage yard in the western United States, and being told "No we don't have one, call Caro" I finally built the corner out of 1/4 inch mesh wire fabric, and fiberglass. Sprayed it black, and called it done. It is the strongest part of the entire shield.
If yours does not have the compound curves like mine did, you should be able to frame up a wood one really easy. paint it and make it look nice, and viola, there you go.