That what I did, it was not a replacement but on a new trailer that did not have an underbelly on it. I went to a local plastic shop, but a sign shop may work too. They had all kinds of plastic sheets, tubes, pexiglass etc to give you an idea.
Bama is right it is called coroplast or something it is a corrigated plastic like cardboard. I actually went with a nice thicker version than the RV store had and it was 7 bucks a sheet, 4 X 9 and took 8 sheets. I looked way nicer on that trailer than my 2008 Wildcat 5er does do to installation and thickness. I have tossed around replacing the underbelly on the Wildcat because it came out so nice.