OMG I can't wait to make this! I had actually never seen it before -- or at least hadn't paid overmuch attention if I did. At any rate, my first thought was also milk cartons.
My second thought is the Shrinky Dink Plastic from the craft store. There are two kinds, one is clear, and one is opaque and textured on one side for using with colored pencils. Though I'd personally prefer a reuse project.