Try a local Builders Merchant, failing that the DIY sheds probably have those plastic ones. The BM's should have lockable steel ones that builders put in / across the bed of their pickup trucks.

Having re-read, my answer assumed freestanding external things, rather than locker-room cupboards.

__________________
Always thank people who have helped you immediately, as they may not be around to thank later.
Visit my blog at http://podsplot.blogspot.com/
I support http://www.hearingdogs.org.uk/