I used the 2-piece Jango helmet from e-Bay. The seller was stretch361 and had a store (Potters Gifts or something). Just under $37 with super fast and free shipping. Cheapest base for one of these I could find. I'm too old for my dad to tell me what I can and can't buy...but I have become a cheap bastage since the economy went south, so I still go with the cheapest I can get away with.
One other fairly cheap thing to use for armor pieces is foberglass/body filler covered wire mesh. I used to build knight armor out of this stuff when I was a kid. Form the wire mesh (I prefer 1/4 inch square myself), cover the back in duct tape, and use the fiberglass or bondo type stuff to cover the outside. The wire is pretty easy to form and shape and you can't tell what it was after filling, sanding, and painting it. Not sure how your parents would feel about you using those kinds of chemicals, but maybe your dad could help out (mine tried to help out way back when...but he was never very good at that kind of stuff and spent more time watching than anything ).
Anyway...good luck to you and hope you get some pics of your progress up soon.