Extruded polystyrene

From CosplayWiki

Also known as insulation foam, it comes in a few colors such as pink or blue, the only difference between the two colors is who is it made by. You can go to just about any hardware store and buy it in varying thicknesses from 1/2" to 2+" thick. It is usually sold in sheets that are 8' long. Make sure that if you are going to paint the polystyrene that you give it a base coat of a latex paint sealant as many things such as spraypaint or fiberglass will dissolve the foam.

Make sure to work with this material in a well ventilated area as the vapors given off when melted or even sanded vigorously aren't healthy.