Cardboard Boxes/Moving boxes needed - pref all the same size (children's party!)

Anyone out there got loadsa cardboard boxes going spare?

It's for a kids party, we're planning to build 2 'rooms' (don't ask - the things we do!) and so needs as many as we can lay our hands on - after doing a trial run we've realised it's so much easier if they're all the same size so that's what we're after. Might be a tall order we know but do post back if you've got anything you think might fit the bill....

Many thanks! Gigi