welcome aboard msudderth!!
i'm not sure if there is any place that just sells the headlight covers. you can always check with the bmw recycling centers. you may not have much luck since you have a newer car.
i will double check with my dealer and see what they say. i do know you can buy the replacement headlight trims, but i don't see why they won't sell the covers. let me check with them tomorrow and see.