A good place to farm Briarthorn is at Wetlands on the Eastern Kingdoms continent. A stack of Briarthorn goes for 5-50 wow gold in the auction house. If you have flying, Cataclysm and of course the wow herbalism profession you should be able to farm 1-3 stacks of this wow herb in one hour.
The best addon to use for wow herbalism is Gathermate2. If you use Gathermate2_Data you will be able to see all the herb node locations for all the herbs in wow.