Thank you everyone

I shall try an "independent" just in case, but it sounds as though the PO is the only (and expensive) option

Seems very odd, as I have a key already - if I were planning to steal someone else's mail I would hardly need a second key? Plus, the majority of people in the USA just have an unlockable mailbox by the roadside anyway.

Still, if that's the way it is...

update - as expected, ACE don't have the blanks, so the USPS is the only option - $25 for a new lock and three keys. Take proof of residential address to the Post Office and they do the rest.