I doubt you will be able to purchase the charger separately. You will most likely need to purchase them all over again. Unless you get real lucky and find someone who has lost the AirPods and is selling the charger.
The charger is simply a lightening cable. Surely you can purchase that separately. Or are you speaking about the charging case? I would hope it could be purchased separately as well? Definitely check with Apple. Be sure to let us know what you find out! I'm interested in this now in the event that something happens to the charging case.