Free boxes can be found at local grocery stores and liquor stores, and other retail establishments may have boxes to give away as well. Mail order boxes make great moving boxes.CheapMovingBoxes.com and Uline.com sell cheap moving boxes, and CheapMovingBoxes.com will even buy back any boxes you do not use.
If you know you are going to be moving, then it is advisable to start gathering boxes well ahead of time to ensure you have plenty. If you have to buy boxes, consider purchasing storage totes that you can reuse at your next location. Ask your friends and neighbors to save boxes for you. Craigslist.org sometimes has free boxes listed if you just go pick them up.
Make sure you are saving any boxes you get when ordering items through the mail. Wal-Mart sells cheap moving boxes, and you can always return those you do not use. U-Haul and shipping stores sell inexpensive boxes in a variety of sizes. U-Haul even has moving kits that contain a variety of box sizes for one set price and a kitchen mover kit to make packing dishes a lot easier. Home Depot and Lowe's sell moving boxes at a low cost and in a large selection of sizes.