I can only assume that it is the same over there as it is in the UK....
Over here it is usually quicker to order a new set of locking wheelnuts, with the 'key' that comes with them, than it is to get a replacement key for the ones already fitted to the car.
All dealers have a master set, so will be able to swap them over for you.
As for someone else being able to buy a key that fits your wheels? If you knew how few variations there actually are of locking wheelnuts you'd be more worried!