There are two locations for you to purchase Celery Stalks in Bulk. These two locations are Wayfarer Foothills (Bjarni) and Gendarran Fields (Milton Book). 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 of these locations I am going to go over is Milton Book in Gendarran Fields since he is the closest one to Lion's Arch and has a ton of different Karma goodies if you're looking to stock up.
Milton Books is a Karma vendor in Gendarran Fields with a ton of different Karma foods available. He is just above Lion's Arch so he should be your first choice if you need an item that is sold by him since the port will cost so little. Milton Book sells Nutmeg Seeds, Lemons, Cumin and Celery Stalks all for 35 Karma each. He's the biggest one stop shop Karma vendor in the game.
The second location to buy Celery Stalks is from Bjarni in Wayfarer Foothills. Bjarni can be found just south of Osenfold Waypoint which is the Waypoint just north of the 1 - 5 Wayfarer Foothills noobie area and/or just northeast of the entrance to Hoelbrak from Wayfarer Foothills depending on how you look at it.