Frost Lotus is typically gathered, in addition to other herbs, from herb nodes in Northrend such as Icethorn or Lichbloom although all herbs of Northrend have an equal 7.5% drop rate.
Wintergrasp and The Conservatory of Life are the only places where players can find Frost Lotus nodes, which require 450 Herbalism to gather, and also have a 7.5% chance to drop an additional lotus.
Players should note that Frozen Herbs will not yield a Frost Lotus.