What is blue foam called, and where can you buy it?

So I was looking at Wolfgang's awesome Mordheim city here:

http://gidian-gelaende.de/Material_HP/Wolfgang/WIPmortheim/album/index.html

I noticed that he uses blue foam, which looks like it comes in thinner portions, and is flexible and you can sculpt it too. I tried searching google for places to buy it, but I couldn't find anything in the US. I'm not sure where I can find the thinner stuff in any local stores, so online is my best bet I would guess. I'm not sure of the exact name I should be searching for. I saw EXTRUDED Polystyrene mentioned a few times, but that usually gets me industrial stuff.

So can anybody direct me to a site where I could buy it in the US? Or does anyone have an alternative material to use? Ideally I would like to find something that is thick enough to make a sturdy wall, but soft enough you can sculpt into it. I also bought some Depron foam, and while it's really great for drawing details into, it's not thick enough to make whole walls. I was thinking of buying some foamcore instead, because I think that is thicker, and you can draw details into it. Either that, or I could just glue the Depron on top of it. What do you think?

Blue & Pink Foam are both Extruded Polystyrene; the Blue is a denser form of the beasty. For thinner stuff I use the "Fan Foam"...Pink Foam in attached 2'X4'X3/8" panels. I get my foam from Home Depot.
On-line you can get bundles of varying thickness foam from the Hot Wire Foam Factory:
http://hotwirefoamfactory.com/home.php .

As DeafNala states above. For large chunks he's using extruded polystyrene, which you can buy at Home Depot (or Lowes). It comes in 1/4", 1/2", 1" and 2" thicknesses. Different sizes are harder/easier to find in different climates (I think it has something to do with building codes in your area).

For smaller pieces (like the walls of his houses) he's using Depron. Depron doesn't bend. For pieces that he needs to be able to curve (like the walls of his towers), he's using Selitac.

The only place I've found in the states that sells those two things (caveat ... last I looked was over a year ago), is a www.rcfoam.com located here.

He sells a sample box for $2.00 to get your feet wet (although what he sends you depends on his mood :-).

There's also Cellfoam 88, which can be found at most HobbyTowns (and I think I've seen it at some Hobby Lobby's). This stuff isn't as nice (it's not nearly as dense), but will work if you just want to try some etching techniques.

Hope that helps!