This Adder's Tongue farming guide will show you the routes I use for farming Adder's Tongue in Wrath of the Lich King Classic.
Adder's Tongue
Sholazar Basin
Adder's Tongue only grows in Sholazar Basin, though it is found in abundance there.
Route 1
I mainly use this route, but if you prefer to farm the whole zone, you should use route 2.
Route 2
This route is basically the same as route 1 but also loops around the southeastern part of the map.