You're going to have a rather difficult time since there isn't any good events that offer itachi susanoo frags. The only surefire way to get him as of now is by pulling from kage treasure. If we assume that 60 seals = 1 itachi frag, then you might need up to 1200 and up seals to put him together. I suggest if you are going to go for 6p naruto, then go for him in lucky stars since that's a good event to get him while also getting seals