University computer labs and the library. They will have boxes upon boxes that originally held 10 reams of printer paper apiece. Good for packing things like books and paperwork: Large enough to hold a good amount, but not so large they can't be lifted after they're full. Also, the cardboard is strong enough that it won't buckle if it's full of shifty-jangly things, like silverware or CD cases, that keep knocking around inside.

Or you could just do what I did, which was take the sturdier boxes to your new home, upend them on the floor, take them back to your old home, and fill them up again.