You can also check to see if the town where you live or a place close to you makes card board boxes. There is a place where I live that makes boxes and they give away boxes and cardboard when you ask them for it. I have gotten some extremely heavy pieces from them. Or you could check at walmart when they stock at night. They have some heavy pieces of card board. Also hospitals are a good place to get heavy boxes. I would try dietary, central supply, material management, and the computer dept at hospitals to see if they have any heavy boxes.
Dang if I didn't post my answer as a comment again....grrrrrr...!!!!
Another good place to look is in dumpsters behind a flooring company. Very very thick display cardboard is used to display tile and carpet samples, and when the vendor changes/updates the sample selection, you can find such cardboard with the samples attached to them in the dumpster periodically.