I always ask callers what they are planning on putting into their ministorage unit so I can help them determine what size will work the best.  Several times a day caller answers with I do not know what I am storing.  I find this to be a really funny answer.

You can purchase a quality lock at most storage facilities, hardware stores, and even discount stores.  I really recommend the cylinder locks.  These rounded, heavy duty locks are very secure.  They cannot even be broken into with bolt cutters because of their distinctive shape.  Cheap locks are okay for high school gym lockers, but are not a good idea on a ministorage unit.