Plenty company's sell the foam padding you need, it's a little pricey in my opinion. You may be better off going to some kind of fabric type shop to purchase and cut your own. It's foam, Ive never done it but, it can't be hard to do. That's just my opinion though.
Classic industries and impala bobs would have what you seek. There's plenty more companies selling the stuff, that's just who I usually buy stuff from.