Ways to get intact demon eye:

For eligible to loot Intact Demon Eye, you must first sacrifice almost your entire life at the All-Seer's Focus altar to become an eredar. This altar can spawn at various locations in Antoran Wastes. By clicking the focus you gain a buff Agent of the All-Seer but instantly loses 90% of your total life. (this may cause you an instant death if you're below 90% health when clicking). While you're disguised as an eredar, with this buff you're ready to go and collect Intact Demon Eye from any mob marked as Demon type. The drop rate is very diverse, sometimes you get zero from 10 mobs, sometimes you get 10 pieces from 2 mobs. While under the disguise effect, you're hostile to all Army of the Light forces, so you may not use their teleport beacons and other assistance without a fight.

Notice that if you die, the buff will be disappeared. 

Excepts pets and toys, one of the exchange item Observer of the resonator when used there will Opens a portal to the Observer's Locus in the Antoran Wastes, Leads to Rezira the Seer who has a chance of dropping Sightless Eye. The portal can be used by anyone in your party.