One of the best places I have found is print or copy stores. The paper boxes are the BEST for packing. One local place I went to over several months provided all my boxes and they were glad to get rid of them. They used the same type paper so often that I was able to color code my boxes per room!
Try http://www.freecycle.com. Its free to join, and many times people give away boxes when they're done with them. I also look behind stores like clothing stores, walmart, Ross, etc. I have never bought a box and I have moved a number of times.