Hammers, materialbutiken was actually cheap... I'm surprised, usually get my "pannå" (that's swedish arty-farty for Foamboard) from NK's thrown prints (they do a zillion a year). They're free, so difficult to get better than that

For the Swedes:
You can get your styrofom (pink/blue foam) from Hornbach and Byggmax for a reasonable price, knowing that most swedish members lives in the woods, the chances are great that there's some retailer within driving distance
