Whether you are selling or moving sometimes you need a REALLY big box. Here are the top ten resources for finding boxes that compiled from the shipping discussion forum:

1. Autobody shops who get windshield replacements
2. Hospitals
3. Sam's Club sells two large boxes and mailers
4. AllBoxesDirect.com, BoxCity.com, uline.com
5. Make it yourself
6. U-Haul
7. Bike store
8. Costcp, Home depot ( their dumsters!)
9. Art supply store, framing shop, furniture store ( dumpsters!)
10. look in phone book for corrugated boxes

Smaller boxes can be obtained for free from eBay and from USPS if you are shipping priority mail.
USPS store and the container store also sell boxes.

Good luck and let me know if you have other ideas!