Model Expo also sells brass sheets and strips, but not styrene. (I wish!) I don't know if they ship to Europe, but they do ship to Canada, so it might be worth checking out.
Edit: Hmmm. Sorry... I think they don't. I just saw on the bottom that Shipping to Continental U.S. I suppose it doesn't hurt to ask, but I don't even see Canada anymore.
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.)