When your long distance movers arrive to pack up all of your belongings, it's a good idea to have them all organized in boxes. Finding enough boxes for all of your things might pose a challenge, here are a few tips to help you with your search.

Friends and Family
Ask friends and family for old boxes from a previous move or purchase that are just taking up space. Offer to clear out their attic, basement or closets to take them off their hands.
Internet
Scouring the internet can yield great results for free boxes. ApartmentTherapy.com recommends searching Craigslist - just go under the for sale/wanted section and type in "free moving boxes."
Local Businesses
Go to local grocery stores and retailers that are constantly new shipments, they will have a slew of old boxes that they're going to recycle. Some of them might already be broken down, but you can reconstruct them with moving tape.
Box Store
There are vendors that offer boxes for sale to people who are moving. They will have a variety of different sizes you can choose from. Be sure to check out a number of sources to get the best deals. Allied sells boxes for a good price here: http://www.allied.com/moving-supplies/moving-boxes.aspx.