There are a number of places to get free moving boxes, including online marketplaces like Craigslist. Since many people often move and unpack their boxes during the weekend, this is the most common time to find free moving boxes.
Some local stores may also offer free boxes to people willing to pick them up in the store. Boxes are generally available after store employees have stocked the shelves. This saves them the hassle of having to crush or break the boxes down. Often the most sturdy and clean boxes can be found in grocery, liquor, book and office supply stores.