Plaques of the Mountaineer are earned from IPP dailies and can randomly drop from killing stuff in IPP including ALL footholds, invasions and rifts. There are 6 or 7 dailies available every day, depending on the day, each one awarding one PotM. The Chancel of Labors has two daily quest givers, Whitefall has one right at the port and the other is located at Exile's Den. PotMs are needed to buy the costume set and you will need a total of 240 PotMs to complete it. The vendor that sells the costume set is located just inside the rear of The Chancel of Labors across from the daily quest giver for killing planar NPCs and right next to one of the two IPP daily quest givers. Scroll to the bottom of the stuff he sells to get to the costume pieces.