Apologies if this is a repost.
The best way i've found to farm Fel Lotus is farming the netherdust bushes on Netherwing Ledge in Shadowmoon Valley. They respawn a lot quicker than normal herb nodes do, and theres a lot more of them when compared to other areas of similar sizes.
The bushes contain the netherdust pollen, motes of mana and the occasional fel lotus. I imagine the drop rate is the same as any other herb node in Outland, but due to the fact that theres more nodes in a smaller space, your chances of getting one are higher.
Netherwing Ledge also contains normal herb nodes too such as dreaming glory and mana thistle.