The Scoia'tael quartermaster is one of the few merchants in Old Vizima from whom Geralt can buy items, if of course he is on good terms with the Elves at this point in the game. The dwarf explains that while he would rather be fighting at the front, Yaevinn has designated him quartermaster. After all, you can not wage war without gold!
If Geralt has chosen the Scoia'tael path, then the quartermaster can be found on the western outskirts of Old Vizima, not far from the Gate to the Dike. On the Order path, the merchant is the Order quartermaster.