How do I find a replacement Taillight cover

My Dad put together His own trailer a long time ago. He is now in His 80's and can't rember what make and model the truckbed was made from. The problem that I have is that the Drivers side Tail light cover is completly missing. How do I go about finding a proper replacement part. The only clues that I have, are that on the sides of the truckbed it says Sport Custom and that the tailgate is a mix matched Ford Tailgate. Not the original that came with the truckbed. Thanks Ben Phillips