EPS is expanded polystyrene, which can mean stuff like DuPont Styrofoam(blue foam 2lb. density) and similar stuff or the white expanded beadboard. If you can, get the foam called EIFS(External Insulation and Finishing System). This stuff is an industry standard, virgin, white beadboard that meets stringent specs for density, uniformity, and consolidation. It may cost more, and it may only be available in large quantities by special order.
If you can't get the EIFS foam, look for any white beadboard <1 lb. density and at least 2 in. thick. Any thinner and it will probably have to be laminated. Ideally the surface of the block will be fairly smooth and not show any grooves or stuff sticking out(regrind). Foam with regrind in it will drive you nuts trying to get a set of nice cores. The really bad stuff has sawdust, wood splinters, cardboard, and other junk in it.