Styrene comes in various thicknesses. Some signs are made out of sintra. If you have a hobby town or some place around you that sells model train kits they should sell styrene. I wouldn't buy it there because those kits places like that sell are way too expensive, but it could give you an idea of what styrene looks and feels like.
most of the 'garage sale' signs are a very thin styrene material. Some of the larger signs can be either thicker styrene, or that corigated sign board material that looks like a plastic version of a carboard box. That is NOT styrene.
I went to a sign shop and asked them for their PVC board scraps. They sold me a bunch for 5 bucks, and it was enough to build my coustume. They were actually glad someone could use it. I build my gauntlets and armor and most of my JetPack with it. The only condition they gave me was to take them a picture of my completed costume.