It depends on what you are looking for:

Fome-Cor uses outer faces (of different materials) and a polystyrene core (I've built large scale, complex 3D, micro layouts with it for photo dioramas)

XPS (Expanded Polystyrene) board is another option. You'll need to do a local search for suppliers, this is what many modellers build layouts on and use for land forms.

Hope this adds to your knowledge.