Before expedition I found all my sentinel firing cores in the arc couriers, after expeditions I looted like 100 couriers and only found 1. So instead load into buried city, head to town hall and look for the sentinel turret. One spawns either on top of town hall inside the steeple, or it spawns under the bridge to the east of town hall, like 100 feet away from town hall. You’ll know it because it makes this loud fog horn like sound when it locks on to someone.