I agree with Fettcicle that Sintra is probably the easiest to work with for the chest, collar, back, and butt plates. All you really need to do is cut out the shape you want, run it under hot water, and <poof!> it is flexible. When you get the shape you are after, just run it under cold water and the shape stays. Mess up? Reheat, reform, and chill it. You 'can' use Sintra for all of the armor, but I personally prefer either vacuuformed or fiberglass for the more detailed pieces (i.e. gauntlets, knees, and jet pack). Being that a 4'x8' sheet of 1/8" thick Sintra generally runs around $25, it is fairly cost effective and shouldn't be too hard to find if you check with sign makers or plastic suppliers in the yellow pages.
