It's alumalite; think of it as coroplast but with aluminum skins and a plastic core:
http://www.signboards.com/products/alumalite.asp
I'm not sure of it's impact rating but it's rated for 120 MPH winds.
A very quick search shows that a 4'x8' sheet currently sells for about $100, so finding a place that will sell you a smaller piece or finding some friends to share it with would be pretty important.