You don't say if you want it rigid or flexible. For flexible, Mully's answer is a good one. Or just get a plastic shower curtain and cut it to a suitable length.
But if you want a rigid plastic one, such as white perspex or acrylic, try a signmaker - they use that sort of stuff for cutting out letters, etc, for shop-fronts and it comes in big sheets in different thicknesses that they can cut to size for you.