Fel Lotus requires a herbalist to gather and cannot be farmed with the traditional method. It does not have its own node to spawn. Fel Lotus can be found as a bi-product while gathering other herbs from the Outlands or it may rarely spawn on a random herb node. So therefore it is rare and in high demands and may be valuable depending on server.
So the only way to gather Fel Lotus is to simply farm any herb from the Outlands that spawns quickly and common, hoping it will spawns and gathered as a bi-product.
See below for maps to gather Fel Lotus in WoW.