Liquor stores are a great place to get cardboard. Find out when they stock the shelves as they literally throw away boxes upon boxes of good cardboard that can be cut up and used.
Indy's CGC Gradings Master Thread - http://tinyurl.com/p45agbh
Want your Comic Graded or Pressed? - http://tinyurl.com/qywwqx5
Notify you of new Gradings? - http://tinyurl.com/no58cku