Julia, I have been on this board a long time. *** ****, or Bob Ryley either, have ever been on it. So don't expect a response from them.
However, the answer to your question is really easy. Look in your Industrial Yellow Pages for Plastics suppliers/distributors. Just about every major city has them. I am in the Detroit area and we have quite a few. Then go visit one or more. Tell them what you want to use it for and they will be able to tell you the type/brand of plastic they recommend for your use. Remember that the plastic distributor is going to take care of his regular volume customers first.
Plastic comes in big sheets like plywood. It can be sawn and even routed like wood. Who is going to install your new window stools? If you feel capable, then tell them the size to rough cut it to. If I were doing it, I would have it cut a little oversized and fine tune it when installing. If you are going to have it installed then have your carpenter tell you what size to order. It will probably need to be 3/4" thick but check your old window. Henry