It is available for purchase from two merchants:
- Legnar in Orzammar Commons for 22 50 0.
- Varathorn in the Dalish Camp (after three main quests have been completed) for 18 0 0.
This is one of the duplicable tomes, along with the tomes for talents, spells and skills. If you intend to take advantage of this exploit, make sure you keep (don't use) the only 2 available Tomes of this kind, either at the original merchant or in your inventory. It's particularly easy to forget about that in this one, since there are only two copies of this tome in the whole game.