There is only a single location where you can purchase Coconuts in Bulk and that is from Lionscout Tunnira who is located in Bloodtide Coast. 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.
Lionscout Tunnira can be found just south of Lion's Arch in Bloodtide Coast. Much like Milton Book she is extremely easy to get to and the port to her will be ungodly cheap since she is so close to Lion's Arch.