Mailbox locks are managed by the U.S. Post Office. To get a new key, you must visit the downtown post office at 210 N Missouri Ave, Lakeland, FL 33815, and provide either a rental agreement or proof you own the unit.
The post office will no longer install new locks for a fee, but will be able to remove the existing lock and leave it open for you to install your own. Most hardware stores sell mailbox locks that you can install yourself.
Owners are encouraged to collect mailbox keys from tenants before they move out so it can be given to the new tenant.