Underbed Boxes that Lock?

Hi all-

Does anybody know of any type of storage container (ideally, something like the large plastic underbed boxes with lids that you see everywhere) that can fit underneath a bed that locks, preferably by key? I need to have something that my children can't get into. I'm trying to contact various container stores, etc. and not having much luck. Would prefer something cheap, of course, as i'm not looking for a true furniture piece as this will be hidden by the bedskirt.

Thanks!