The best material for envelope

Hi,
My name is Princo and I'm from Wroclaw University of Tewchnology.
I'm designer of this project:
http://airshipworld.blogspot.com/2008/04/airship-one-project-from-poland.html

and now Me and my  team are preparing to build out door airship with full autonomical control. My question is: What is the best material for envelope and where can I get it?

Plese help