I know that some folks have used that green floral foam to get the basic shape of their project. It's the stuff they use for doing silk flower arrangements. It comes in different sized blocks, and you can deform it easily with finger pressure, or use different cutting/carving tools. I'm sure once you had it carved, you would have to put some kind of coating over it to make it keep the shape.
I've never used this myself, so not sure how effective it would be.
I think you are looking for polyurethane foam. I looked at the link above, and the sculpture foam seems like what you are looking for. The foam comes in different densities. The foam on that site is 2 lb density. That is rather light. I use 4 lb density for most sculpts and like 6 lb density for smaller things like helmets.
The first place to try is look in your local Yellow Pages under "Insulation" There are places that sell it in sheets of 2", 4", and 6". I have a local source that has 4'X4'X3' buns, and runs it through a really big band saw and will cut to any size.