There are two locations for you to purchase Horseradish Roots in Bulk. These two locations are Sparkfly Glen (Laudren) and Fireheart Rise (Wupwup Chief). 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 location I will go over is Laudren in Sparkfly Glen. Laudren is right nexto Dryground Waypoint in the northern area of Thundertroll Swamp which is located in the south-central area of Sparkfly Glen.
The second location to buy Horseradish Root is in Fireheart Rise from the Wupwup Chief who is located in Apostate Wastes. Apostate Wastes is in the southern end of Fireheart Rise, the closest waypoint being Apostate Waypoint. There are two different Renown Karma vendors on my map for this location, the Wupwup Chief is the one further to the south.