Major in Georgetown has a bunch of officially licensed New Era caps, including a few exclusive Nats designs. They're appointment-only however and the atmosphere can be kind of intimidating if you're not used to that type of streetwear boutique.

Other streetwear boutiques like Atmos, Commonwealth, or A Ma Maniere might carry Nats hats as well, but none of them are originally from DC. (Commonwealth is based in VA I believe.)