Stopped by FastSigns today, and they sold me the ONLY sheet of PolyStyrene they had, which happened to be 48"x96" in White. It was a lot more than the Black sheet I ordered ($65.00!!!) but I didn't care, because I want to get started on this now.
Had a nice chat with the folks that work there.
Before I found FastSigns, right down the street was a smaller sign shop called SignSource, which is privately owned. I had a long chat with the manager, Mark Williams, who was very knowledable about the material, even though he didn't stock it regularly.
He explained that Styrene is typically used for cheap throw-away signs for promotions, when a quick and dirty solution is necessary. He says it has a short shelf life, and brittles horribly due to temperature and light exposure (even indoor lighting). It has a tendency to yellow, as well, and he asked me a few questions about why I needed the material. I explained the basics, and he recommended I look into an industrial plastics supplier by the name of Industrial Safety Supply, who may have sheets of ABS plastic that would be suitable for things like American Football/Baseball helmets, as it is a lot more robust, but is much more expensive.
For now, I think I will stick to the plan, and use the Polystyrene, since the helmet will be painted, reinforced, and glued. Mark wasn't sure about the malleability of the thicker ABS under heat, but thought it would probably be about the same, but recommended I talk to ISS, since they deal with the materials constantly.
I'll speak with the folks at ISS later, but for now, I have more than enough Polystyrene to make a bucket, and probably the rest of the plastic parts of my armor. Not to mention, I still have another 48"x96" sheet on its way here, in fact at the moment it's in Ohio.