Mailbox keys are often small, hard to keep track of and easy to lose. It's best to attach your mailbox key to the rest of your keys when you first receive it, but if you haven't done so and yours has gone missing, you need to find a replacement quickly. The ease and expense of doing so will depend on a number of factors.
Contact the building manager or caretaker to see if they have a copy of the key to your mailbox or a master. If they do, you can borrow it to open your mailbox and ask if you can take it away to get another copy cut. If you live in a large apartment complex and have had little contact with the building manager or caretaker to date, it's a good idea to take along some proof of residence.
Call your landlord or the real estate company from which you bought your property. Your landlord could well have extra copies of all keys for your apartment and should be happy to provide a replacement. It's less likely that the real estate agent will have retained a copy, but it could be worth a try.
Talk to your postman. Some postmen have master keys for large apartment blocks to speed up their work if they are delivering large amounts of mail to one person. They may be able to help you get into your mailbox, but will probably not be able help with a replacement key.
Search online or check your local business directories for a locksmith. When all other options are exhausted, you will need to pay for a replacement lock. Clear it with the building's manager or your landlord before you have a locksmith come in to work, as the mailbox is most likely the management's property, irrespective of whether or not you own the apartment.
Attach the key for your new lock to your keyring or put it in a safe place where you're unlikely to forget it.
If you need to have a replacement lock fitted, make sure you have more than one key cut for it and give a copy to a friend or neighbour for safekeeping.