There is only a single location where you can buy Avocados in Bulk and that is from Fallen Angel Makayla in Fields of Ruin. 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.
Fallen Angel Makayla is found in Ebonhawke which is the large town in the southwestern end of Fields of Ruin. There are two Karma vendors here, the one you want is the Karma vendor in the center area of the town not the one in the southwestern portion of the town.