Seal Treasure. Or buy fragments at the black market, they pop up from time to time.

Does he ever show up in arena/group victory/survival/ranked store?

Itachi may show up in a future update to the exchange merchant. In the meantime, you'll have to rely on luck in Seal and BM to get fragments for now.

Itachi might show in the Lucky Ninja Pack or something

There's a few main ways to get Itachi, however the most common is usually buying fragments, or if you have Naruto-level luck, the Seal Treasure. The main places fragments for Itachi will possibly show up is either what Kfactor stated, about how it'll possibly be in a future minigame/ update, or through the Black Market Merchant. I'm pretty sure there's no instances where you can get Itachi, or any Akatsuki member fragments at all, without either being lucky, want to spend a LOT of money on getting seal scrolls, or just being patient. :victory:

I got him from the seal treasure on a free pull. Just pray before you pull.

I want Itachi so much lol. Im very unlucky, at seal treasure i only got choji and hinata

REALLY WANT ITACHI SO MUCH HE IS MY FAVORITE CHARACTER

You can get him now in arena shop :curse:

Ah, he's in Survival Breakthrough. 2 to 4 frags per draw I think.

If you cannot wait, use your Arena credits to exchange for his frags. He's kinda pricey thou.

He's also a common rare in Kage treasure, along with his partner Kisame. I've gotten him twice in that treasure now.

And if you got RNG-sus's blessing, you could get him from wheel/ninja pack. That's how I got mine.

He is a common rare in kage treasure. In 360 draws there i got him two or three times.