Kwatermistrz Scoia'tael is one of the few merchants in Stara Wyzima from whom Geralt can buy items, if of course he is on good terms with the Elfów at this point in the game. The dwarf explains that while he'd rather be fighting at the front, Yaevinn has designated him kwatermistrz. After all, you can't wage war without gold!
If Geralt has chosen the opcja Scoia'tael, then the Kwatermistrz can be found on the western outskirts of Stara Wyzima, not far from the Brama na groblę. On the opcja Zakonna, the merchant is the Kwatermistrz Zakonnu.