I was just made aware of Frogbox: a neat business idea brewing in Vancouver – provide a service to people that includes reusable moving boxes, a dolley, and pick-up/delivery at your house.
They even have a sustainability-related conservation message/donation policy!
If you’re moving in Vancouver or Seattle, I’d suggest checking them out rather than looking around for old carboard boxes. They’ve taken the sustainability idea all the way, including bio-diesel powered trucks to deliver and pickup the empty boxes.