Nelson, do you know a shop for R/C airplane models? Perhaps you are lucky and find styrene plates there. As I remember, Graupner (a German model airplane manufacturer) sold styrene sheets in several thicknesses (about 80 x 20 cm).
If you want to order from the USA, of course there is still
Walthers (searching for 'styrene sheet').
(Perhaps there are cheaper sources, but so far I never had postal problems with parcels from Walthers.)
Ron