Im pretty sure it drops from the court of truth instance that you get access to after you finish all the truth quests, you kill the same sphinx in there again and you can do it every 3 days, ive done the instance a few times and no luck yet.
i dont think u can spawn him again if u finish the quest sadly, iv tryed and had no luck, also rumors on AB say that the shield dosn't drop anymore, true or not i dont know. hpe it does still drop though.