You should be able to buy that foam at a good foam/rubber supply house that is frequented by upholstery folks. It comes in a number of colors, feels waxy and is closed cell stiff yet somewhat flexible. They will cut it from a block to make a sheet to almost any thickness.
BTW, some packaging uses this stuff for fragile stuff, I keep it all as I always need a bit here and there. In fact used some to fill the toungue hole under the hitch on my boat trailer, keeps dirt out and most importantly it keeps the wire harness from pulling out. Some old boat cushions that float/throwable have it, the outside wears out before the foam does...salvage it. Just ideas.
Good luck it really does the job. Hey you'll do just fine...really sharp knife is best.