I bought it from this site: http://www.snmstuff.co.uk/
Just go on products -> modelling materials and you'll find it there.
There are many different types, the one I use is the standard one, it's very cheap and very much material.
It's gonna make the 35 bases of the base set easily.
It's a nice site, with very low shipping costs, I could buy my paints from them but I find them cheaper elsewhere.
But I've bought sculpting tools, files, brushes and other stuff.