I'm guessing this is the same stuff as Kappa board, which is used in just about every design studio in the country for mounting pages of artwork to before sticking them in a big portfolio case to take them round to client presentations.
It's most commonly available in black (although the foam core itself, which you'll see on cut edges, is white - it's basically a layer of styrofoam sanwiched between two sheets of card).
You should be able to get it from any art supply shop.