This place is phenomenal if you need any types of plastic pieces - I use them as templates for sewing projects and have them cut pieces for me. They're usually able to cut everything within the hour. (Probably could do it faster, but since I work nearby, I never ask them to speed it up.)
They have sheets of plastic in multiple colors and sizes. They also have vinyl - if you want to make your own shower curtain, or pockets on a beach bag, for example.
They also sell glue so you can attach plastic pieces together (you need special glue, and an applicator that applies it perfectly.) I made a little shelf for my bathroom in this manner.
They also sell small circle and square pieces of precut plastic.
Definitely your one-stop for plastic!