Styrene is what most (not all) modern kits are made out of, it has great molding caracteristics and is relatively easily worked(sanded, cut, bent etc) It is also easy to heat form as in Vac form kits.
styrene (not to be mistaken for expanded styrene foam) is also available in stock form in varying architectural shaped cross sectioned lengths (used by architects, railroad modelers and millitary modeling enthusiasts etc) it is a favoured modeling material because it is made of the same substance as the kit itself and can be handled in the same way as the kit parts (other substances require special glues and different finishing tecniques than styrene).
It is also available in card form and in clear, indeed a lot of aircraft builders vac form there own canopys from clear styrene.
Evergreen are the largest supplyers and most shapes and sizes are available. Most good LHS stock a good supply of Evergreen styrene products.
I hope this is of some use to you, Gotter