If you live in a smaller town it might be more difficult. Walmart is not a good one to work with I have found, so I found an action figure selling warehouse that sold stuff on ebay (that was an interesting business find!) for my last move in this tiny town. Drive around the back of businesses and see if you find piles of broken down boxes to get an idea of who generates a lot of box trash.
I always asked the manager before taking boxes from the dumpster and most were congenial enough to save them up for me. They generate a lot of box trash when they receive shipments.