Cerberus guild

Firstly, I see them on the backs of people but everyone I've asked has bought theirs so I have no idea where they are found. Does anyone know where to get one before I request assistance in grabbin one?

It drops of Miller in Adonis, one of the bosses in the unredeemed dungeon there. kill the placeholder, spawn Miller, he might drop Heirloom of Eva Miller, which turns into the backpack.