When I moved recently, there were a few places that I went.
Try:
U-Haul- they sell them... they are a little expensive
Big Lots (pic'n'save)- cheaper...
Goodwill
Home Depot- if you ask an assistant manager, they will tell you that they "recycle them", but if you ask the store manager, telling him/her that you are a good customer and will continue to give them business if he/she will help you out, I bet you'll get all you need within 2 days.
Don't give up! Free boxes are everywhere! Wal-Mart, Target, Lowe's, Home Depot, Circuit City, Vons, etc!!!!!!
james