Look for expanded polystyrene, that is what your typical white Styrofoam is. You could also use extruded polystyrene if you aren't planning on melting the foam out with acetone.

Hi,

I am going to make a Styrofoam mold, but after cutting the foam into the final shape, since the structure is so thin (9mm thickness) i'm afraid it's gonna break wrapping the layers over it. can i coat the surface of the Styrofoam with gelcoat or Glasscast to make it stronger?