where can I get used cardboard beer cases?

Liquor stores! Here in my neighborhood in Denver there are three I stop by when I need cases for my bottles. I've asked a cashier or a manager if there is a good day of the week to collect them, and then I try to stop by as close to that time as possible. Sometimes I'll even get them before the boxes have been broken down by the employees, and I won't need to tape them back together myself. Never had a problem getting them for free.

Umm...a bar or liquor store? An Army PX, college frat houses. Bottling plants may have some too beat-up for regular use that will be okay for storage. Look for related storage devices online; no doubt something's being manufactured for people with similar needs to your own.

If you had fantastic luck, I suppose you'd be there waiting at the custom bus rental when a heavy metal band rolls in at the end of a tour : D

Frat house!

Liquor stores usually have them lying around. I'm sure they'd be glad to give them away.

Honestly... I always had the best luck asking a bar owner !!

GOOD Luck.