You guys really should check with sign shops in your area. I get 4x8 sheets any color for $12 from a sign shop here. I showed them a picture of what I was doing and I've even taken planes in there to let them know I'm not a competitor buying materials from them and then selling signs.
If you just want to build 1 or 2 this size, those sheets at HD & Lowe's are ok I guess. If the bug bites, you're gonna need to buy in bulk.
