Polyfilla

Does anyone know where I can purchase Polyfilla to be used as a groundwork medium? And how is it applied?, thick or thin?

You don`t say where you are from. In the UK, Polyfilla is very common. It comes in many forms, from dry powder (to mix yourself), to ready mixed in a tube for filling holes/cracks in plaster.
I prefer dry-wall adhesive for groundwork, same smooth texture as Polyfilla, much cheaper ( a bag will last forever) and has some flexibility so it wont crack.
Hope this helps.

Iam not from the UK, I live in the States but I am currently stationed in Iraq. I read about Polyfilla from an article in Military Modelling. 2 guys used it as their groundwork. But the one built a diorama of a destroyed Panther in a church, and from the top of his base to the actual ground was varying in depth from anywhere from 2-4 inches. Is this all Polyfilla? Or is he covering styrofoam with Polyfilla?

He will be covering foam with Polyfilla, I only use it up to 1/2 inch thick.

Thanks for your help, my groundwork is an area of my modelling that I need to improve upon. It always seems to come in last when I build a kit, but I think that it is just as important. Would you happen to have a site in the UK where I can order some?