Ah, Evergreen's been mentioned. Another good source of plastic for use in modeling would be certain types of plastic containers.
How many times have you seen the coffee creamers in a restaurant? The containers are made of styrene plastic and are usually very thin-walled. These containers have helped me out a couple of times when I needed very thin strips of plastic and didn't feel like sanding down a strip of Evergreen.
Any other styrene plastic containers are also useful, though many are not of uniform thickness or quality. But if all you need is to fill or shim a gap or some such, then this stuff is ideal and most times, free. Many microwave dinners have styrene dishes. Look around the house, you may find something useful!
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