Orochimaru's organizations, based on whatever information we can grab from Leaf/old contacts/our history.
Get J's bois on it.
Lets go the whole 9 yards too.
Assuming Akatsuki is splintered and we have to contact most of them individually, here's how I might go about starting the brainstorming process of doing that (beyond vague "put feelers out" etc.).
1) Contact Kakuzu
-- There must be a way for him to satiate his lust for monies with bounties.
--- The pile of gold in our treasury is looking a lot more expendable at the moment, by the by.
2) Contact Hidan
--He is the religious leader of a major cult and might have gone back to get stuff done with said cult.
--- Noburi
knows where this was situated.
3) Contact Konan
-- She's an old student of Jiraiya's, met during one of the wars. We know he fucked around in Rain a bit, so maybe she was from there and went back there.
--- Ask Tsunade to confirm this backstory even.
-- We have reason to believe Akatsuki Guy was in Rain for extended periods (Take a linear combination of plausible reasons). So maybe she or someone else is there.
4) Contact Itachi/Kisame
-- Summon realm.
--- We could ask Sasuke maybe about the formers whereabouts under the guise of "Hazou the Righteous Moron has a major grudge to settle because That Man and His Partner helped kill our dad."
----Perhaps he can point out intel on Uchiha Temple.
5) Contact Deidara
-- Kagome said "He was the major supplier of explosives in the Fang/Claw war." so they might know a way to get in contact with him if this is true.
--- Alternatively start blowing stuff up.
6) Contact Sasori
-- That Wind village clearing mission might have been him (he would know the area and have the means to easily clear land for miles with his kajillion puppets)
-- Maybe Chiyo knows.
7) Contact Nagato
-- See Konan section.
8) Contact Orochimaru (Dead or Alive)
-- Basement is a nice start on leads.
-- Get in cahoots with his organizations, see if we can piece together where his Horcruxes are.
9) Contact Zetsu (Assuming he exists and/or Zetsu = Lupchanzen)
-- Follow up on that Kagome interlude where he talks to the weird merchant guy.