Chipboard is nice but expensive, and I've never found where to get it for free.
You can get cardboard free lots of places though. I have had luck in supermarkets: Aldi has some big sheets they use in their pallets, and Meijer just told me to take any boxes I liked if I see piles of them when they are restocking (Sunday nights usually). I called some mall anchor stores and Sears gave me these nice HUGE sheets from refrigerator boxes.
I also tried hardware stores and furniture stores, but they all seemed to break down boxes and throw them out right away.