High density foam is probably going to be easier to locate than Renshape.
There is a product called "Balsa Foam" out there that is essentially a chunk of Urethane Foam. The stuff is like green florist's foam, but tougher (you can't easily put your finger through it). It can be cut with a variety of tools (X-acto blades, small saws) sanded down to size with coarse sandpaper, and intricate shapes can be carved with a wooden stick or even a toothpick. The stuff is essentially the same foam professional model builders use when they want to carve something quickly and have a relatively smooth shape. The surface is pourous and you will need to seal it with a layer of Bondo or resin if you want a smooth finish. If you don't mind a bit of texture, I've used Gesso or Bondo Glazing Putty to seal it up. This stuff is great for replicating stone or rock.
The different densities available refer to the weight of a cubic foot of the stuff. The higher the number, the more dense it is and the less you will need to seal it. Be warned that the stuff will rust steel X-acto blades and saws like nothing I've seen if the dust isn't cleaned off. And the dust gets everywhere. Definately do this outside (preferably in the neighbor's yard).
Renshape (and I think there is a brand called Model Plank) is a much denser and stronger material. While you can get some fairly intricate shapes with the Balsa Foam, it will be somewhat fragile. Renshape can be put in a mill and product designers use it to make prototypes for all sorts of things. Also, a couple of good coats of primer sanded down will seal it. It might be tougher to carve (you'll need more than a wooden stick), it is easier to work with than, say.... plexiglass.
Either case, get a respirator or a really good dust mask.
I've found Balsa Foam at well supplied hobby shops and craft stores. Renshape is more of a direct order type item. I've gotten by with the scraps I have left over from jobs, so I don't know where I would get any.
Hope this helps.
GK
