I used posterboard I purchased at wal mart, more expensive than cardboard which is readily given away free. But you can run it through a printer and print up your layout directly on it.
Another option alot of the 501st star wars guys use is sintra, its that plastic stuff for sale signs are made of. It can be bought in large sheets for little money, and is fairly easy to work with. I reccomend using AC glue found only at hobby stores as it causes a type of plastic welding when it cures.
as far as bonding agent for cardboard, I would reccomend, bondo (the green stuff not the red) its easy to smear on, and sand down, and will make a nice helmet when completed. And its hard and durable. Fiberglass it messy and takes along time to cure.
I hope this helps
DM