Two of the biggest suppliers of styrene in various forms are Evergreen and Plastruct. If you browse around their sites, you'll get an idea of the range of products they supply... and what you might be able to do with them.
i am gonna go make a visit to my hobby shop. it is like for planes and trains and such, but they may have something.what would be the best thickness for a e11 sights and stock? and possibly um...argh cant think right...o... mag housing and mag. like...1/16 of an inch?
In my experience, 1 mm and 2 mm are pretty good for "general construction". Use the 2 mm for structural parts, and the 1mm for curves and detail. You can heat form 1mm styrene with a decent hair dryer. In your example: I would 2mm for the stock and 1mm for the sights.
Styrene is dirty cheap. Get several different gauges and see what works best for you.