Once unlocked from the inside you can remove the lock from the door. You can then take it to a lock Smith to get a new key made. Some of the locks have the key number etched on the inside. Once you get the key number you can order new keys.
If it is an FIC lock you can order new lock cylinder cores and lock cylinder core removal key and insert new cylinder cores. They are available on ebay and Amazon. The cores and keys are sold in sets. The cores are replaceable just like the cores on Yakima roof racks. FIC and yakima use the same double sided key blank design. They may even use the same lock cores.