I just went to Pep Boys and bought a reflector to harvest the lens material. I think I had to to buy 2...1 for each side. I used an amber reflector since it was going on the front in my case. Look around in their trailer section.
Try an electrical supply house and ask for prismatic lense material. It comes in different size prism shapes, like K12 prsimatic, etc. It's usually clear, so you'll need to color it once you get it done.