Anyone know a good source for boxes for speakers?

In NY metro, Boxes are everywhere. Look on any street corner or retail shop- especialy on recycle days....

I second the U-Haul suggestion . They make a fairly large box called a wardrobe that I used for some JM Lab Electra
926's (mentioned for size comparison) . That coupled with some 2'X 4' strofoam insulation sheets from Home Depot made for a very sturdy and protective shipping container . The two wardrobe box's and two packages of the styrofoam with a role of heavy duty box tape was about $50.00 total . I have used this method twice and received nothing but praise for the packaging of full size floor standing speakers that were shipped across the country by Fed X .
Good luck !

Skip the cardboard, it will not help you if the speakers are really abused as they often are when shipped ground.

Make some crates from 1x4 rough cut lumber, add 2-3" on all measurements, line crate with cardboard inside wood and then foam or bubble wrap, wrap speakers in plastic or cloth and put them in. This will be much more secure if you are shipping them UPS or the like.