Check out large stores in your area such as Wal Mart or Target. They have all sorts and sizes of boxes where new products and merchandise are contained before displaying them on the shelves. Most of these boxes are just thrown away so you might want to ask the store staff and they will be more than happy to give them out for free. Just ask nicely and everyone will benefit.
Specialty stores are also a great source for free boxes. If you are shipping oddly-shaped items like a surfboard, stroller or fishing rod, go to shops which sell these items and more often than not, they have boxes that they just dispose of.
Before packing your stuff in these pre-used boxes, examine them first for damages, holes, etc. Store staff usually disassemble and fold these boxes for easy handling and storage. Sometimes, boxes can get damaged during the delivery of items or while the staff were opening the packages with their handy cutters. It is a good ideal to support the seams and flaps of the boxes with packaging tape to give the box additional strength.
Aside from saving on extra costs, re-using these boxes also prevents otherwise good cardboard shipping boxes from going into the trash.