Hmmm, you mean the stuff that can be crushed but springs back?
That is EPE or Expanded Poly Ethylene.
https://www.google.com.au/search?q=E...sheet&tbm=isch
You should be able to get sheets of it from any foam wholesaler. Try to steer clear of places like Clark Rubber who usually apply a huge mark-up.
Polystyrene (EPS) is brittle and doesn't regain its original shape.