If you need a small section, find a local sign shop. It is the same plastic they use for yard signs. They will have it in white, but may have some in black. They may even have a scrap piece or a misprinted sign they will give you. But they should have the same sort of plastic. You can spray paint it black if the color difference matters to you (and they don't have black).
Or check with your dealer where you bought the trailer. They may have a small scrap from a repair job on another trailer. It's trash to them if it's a cut off piece, but you only need 4" or so, right?
Just the 2 of us in a...
2015 Freedom Express 246RKS
2002 GMC Sierra 1500