Shisui needs to be in assist spot for itachi. And you need level 25 between the 4 ninjas on Anbu Itachi. You dont need shisui on his team to keep the bond mystery as long as you has shisui in itachis assist bonds. I buy purple threads now that I am finished with getting the itachi bond. Keys are easy and cheap in every event they are in. Adv Thread is hard to get. Adva refines are best what i would buy and what i am buying now that I no longer need scrolls.