In chapter 2 you can buy a diagram called "Diagram: Draug armor" from one of the merchants/craftsmen in the Kaedweni camp. I did not buy this diagram because I didn't have a part of the Draug yet and I was not sure if I might have missed that enemy already (or forgot to loot it). I wanted to save money so I waited to buy this diagram.
Spoiler (hover to view text):
At the end of chapter 2 I encountered the enemy "Draug", killed it and looted its fragment which is needed for the Draug armor. Unfortunately, I was not able to buy the diagram anymore because the Kaedweni camp was left by the troops to march on Vergen. Now I have the Draug fragment but can't craft this powerful armor because I don't have the diagram.
Is it possible to buy the diagram after completing chapter 2? I went to Loc Muinne and visited the merchants there, but could not find the diagram...
In case it should be impossible to get the diagram in chapter 3 can anyone recommend another powerful armor whic his obtainable at this point of the game?