We broke one of our rock guards also and I worked at determining if I could get the plastic. After some work, I determined that it is available as a sheet, but I had to buy a 4 x 8 piece. The aluminum frame can be taken apart. You can pull out the old one and use it as a template. But the old (original one) is curved and the 4 x 8 sheet is flat. So you would have to flaten the old one for use as a template, then bend the new one. Probably could do that with a heat gun, or possibly just let it lay out on a really hot day. I ended up just buying a replacement from Out of Doors Mart for a couple of hundred dollars. The sheet of plexiglass was going to cost $100 or so all by itself.
BTW I still have the aluminum frame and broken plexiglass if anybody wants it. It is the street side guard.