There are two locations for you to purchase Black Beans in Bulk. These two locations are Kessex Hills (Deputy Jenks) and Gendarran Fields (Field Medic Leius). Now before I get started with telling you the two locations keep in mind in order to purchase any goods from a Karma vendor you must first complete the quest they have for you. That is of course if they have a quest available if not you'll be able to buy whatever they're selling right away.
The first NPC I will go over is Deputy Jenks who is located in Kessex Hills nearby the Overlake Haven Waypoint (southeastern end of the zone).
Deputy Jenks has quite the selection of Karma goods for sale. Not only does he sell Black Beans but he also sells Nutmeg Seeds and Bananas in Bulk. It's something you'll want to keep in mind if you need more than one Karma item and you want a one stop shop type of NPC.
The second location where you're able to buy Black Beans in Bulk is from Field Medic Leius who can be found in Gendarran Fields, the closest Waypoint is Talajan Waypoint or Nebo Terrace Waypoint.