The only mob that dropped this for me after clearing the north, west and south wings multiple times is a mortiferous golem. It is located on a ramp from the southern section leading to the east section. You need to travel through the south section, staying to the east until you come back close to the center plaza. Beyond the ramp is a one way wall into the east wing so if more than one member in your party needs the drop hang around for a respawn. I have only partially cleared the east section so I do not know if there are any addition mobs that drop this.