This Felweed farming guide will show you the routes that I use for farming Felweed. There were no major changes to herbs at Outland, so this guide can be used for Burning Crusade Classic (TBC) and Retail WoW.
Felweed can be found in every zone at Outland.
Blade's Edge Mountains
The best zone to farm Felweed is Blade's Edge Mountains. In this zone, you will mostly find Felweed, Dreaming Glory, and a few Nightmare Vine.
Nagrand is also a great place to farm. Just like in Blade's Edge, you will mostly find Felweed and Dreaming Glory and nothing else. This is why these two zones are the best because there is no other kind of herbs here.