looking for clear sheet plastic

Plastruct, Evergreen and Tamiya(Pla-Paper)make some, but I am sure there are more companies producing.
I use them a lot for making dial faces and similar.
Thinnest I have used is 0.2mm but they come in different thicknesses.

They are also nice for vac-molding new canopies and similar.

Check your LHS, Arts Supply and local Model Railroad Stores for architectural model supplies.

I think Micro Mark sells it, and also squadron.

www.micromark.com
www.squadron.com

If you can't find or don't wish to purchase clear sheet as suggested above, you could use the center sections of the clear plastic tops to the aluminum trays that many Chinese take-out restaurants use. In addition, take a brouse through your local super market. You will be surprised at how much of the packaging use these clear flat thin tops. I've even used some of this stuff for vacuum forming new canopies.

Pete

Squadron sells a clear they suggest using for thermoformed canopies.

I have used the clear plastic from "bubble" packages and pasta boxes to make reflector gunsights for 1/48 aircraft. If you need a certain shape, check out the packaging on some products in the supermarket or pharmacy. Might have something there you're looking for.