I paid $70 for 13 square feet of 3/8" thick black HDPE. It's a little more expensive than the standard hdpe, but it's uv stabilized so it won't become brittle if it's left in the sun for a long time. I bought it at
precut in 4x1 pieces (I also bought a 1x1 square), and shipping was $15. The hdpe came with a plastic film over it to protect it from scratches, and had a very smooth finish when the film was removed. I'd recommend buying from them if you cannot find a local shop to supply you with plastic.