Where in Minnesota are you? If your looking for corrugated I know there are at least two corrugated plants in St Paul and Minneapolis. Look for "Smurfit Stone" in your phone book and give them a call, ask for the Plant Manager. If you are really nice he'll save you a piece. Just tell him the size and what you want it for.
The plant managers get calls all the time, mostly from schools and such, wanting pieces of corruged for craft projects.