Home of the Sovereign Protectors

Hey, so I'm setting out to actually mold and cast my helmet, but to start I need to make my master prop, and I'll be using several stages of foam, paper, and tape. First thing's first, I need foam insulation panels to glue together and sculpt-down to half of my helmet. What... like, what size would be suitable, and where could I get said size?

There can be found a post here where you get more Information about the helmet.
http://forcepike.net/forum/viewtopic.php?f=34&t=1257