I hired a carpenter to do mine. It's a combination of ripped 2x4s and arched 2x4s. So the material comes from literally any lumber yard. The arches are made cutting the 2x4 to the correct length, and then finding the dead center of them, then measure 7 inches out from dead center, and then mark it. Then measure 7 inches out the other side, and mark it. Then on the ends, measure the height of the sidewall, minus thickness of your decking. Mark that, and then draw a line connecting the two points. Then cut and smooth it out.