The owner of the box is responsible for their own lock, not LSIA or the Post Office. The Farmington PO has, however, agreed to replace locks for a fee of $20+. You’ll have to contact them to make the request.
If you want to save some money, you can buy your own lock for $7-8 and do it yourself. If you can still get into your box, you can easily switch out the lock. If you can’t get in, you’ll need to catch the mail carrier or call the PO to coordinate timing and he can remove your existing lock from the inside and then you can put your new one on. It’s not difficult, but you need to make sure the door is snug when locked with no open gap for rain to get in. You might want to try using the lever from your existing lock if the ones in the new kit don’t work.
This is what you need: Home Depot: Mailbox lock or Lowes: Mailbox lock.
If you want copies of your keys, HD and Lowe’s don’t carry them, but you might be able to get copies at Aubuchon, True Value or Ace or contact a locksmith.