Justin:

Extreme Panel (near you) will probably sell you blank panels. Consider the waste issue, though. When you cut your own EPS panels on site, you get a LOT of little styrofoam beads everywhere. They blow all around your site, and into your neighbor's yard, and they won't degrade. And your panels scraps - what do you do with those? You'll need to burn out all the recesses for your 2x bucking and plates - STINKY! Probably not good for your health either. If you have them custom cut in the factory, all those issues are taken care of for you.