Food supply store - pectin
Pomona sells pectin in larger than single batch quantites but I have no idea if it's available in the DC area. Contact info is at website:
You might ask the moderators to switch this posting to another forum where you will likely get lots of suggestions.
If you're looking for mail order or internet sources, as well as local DC/Baltimore sources, please do start another thread on General Topics to ask about them. That'll keep this thread focused on sources in the local area. If you're really interested in local places where you can buy pectin then your thread is already in the right place.