Where to go for a replacement mailbox key?

Crap. I discovered last night upon coming home that I've lost my mailbox key. I've already torn my bag and my apartment apart searching for it with no luck, so I'm guessing I lost it in the city or on the subway somewhere.

Does anyone know whether getting a replacement is something you have to handle with your building management or with the post office? I've left a message with my building management's answering service, but they're unlikely to get back to me before Monday.