Got these on a named mob in a Oasis advent. Can't remember the mobs name but was a caster for sure. 51 was our highest party, these sure beat the dwarven work boots i been wearing for how long.
Dropped for me in a Collect 28 dungeon off a mob by the name of "Takish-Hiz Protector"
Was a pure melee mob, he didnt appear cast or try to cast any spells but did have a DS up on him when he was encountered and i dont recall a caster being pulled from the room so he could possibly spawn with it up.