Most of the bottle shops around here have stacks of those boxes at the front of the store. Whenever I'm in there, I buy a six-pack or two and, when they go to put them in a bag, just say "how 'bout a box". You could probably just ask them for some anyways as they probably have more of them than they know what to do with.
Another option, is copier paper boxes from work - there always seems to tons of them being discarded. Four six-pack holders fit easily in those and they're plenty sturdy.