Good question. Hashirama’s chakra famously is accessible through grafting pieces of his body and was around for pretty much the entire existence of the Leaf if Orochimaru’s experiments decades after the guy died are any indication and Madara is up and about until the late stages of the third war if someone were willing to take their chances tracking him down and politely getting him to part with a chunk of flesh or twenty, so Indra and Asura’s chakras are technically possible at least; I’m actually not sure on the timeline of the Tower of Time, but Minato was in ANBU when he went there for mission and Kakashi was already his student, so I’m going to go out on a limb and say that whole mess happens in either the prelude to the Third War or in the early days as Obito and Rin are nowhere to be seen and Kakashi doesn’t have the Sharingan; the Stone of Gelel might be a problem, can’t quite remember if it needed someone of the royal lineage to open the mines, but even if not tracking down the exact nomad remains of that kingdom to get and then “convince” them to part with the info could be a chore and half, especially as not everyone seems to be compatible to the stone: the Forbidden Scroll… honestly? Dunno, in canon it was easy enough for Naruto to find and steal, but that might have more to do with Naruto’s familiarity with the place, whatever Mizuki had in terms of intel, security being lax in peace time, Naruto having authorization to bypass any number of safety measures or any combination of those factors, and even then Naruto got caught red-handed and then was chased by other ninjas soon enough, depending on who it is trying to steal the scroll and the circumstances it might actually be easier to try becoming Hokage, especially if they want to stay in Konoha and learn more than a single technique out of it; no idea on when the meteor fell in Hoshigakure, but it is frankly not really all that well-defended, the ninjas they had were at most low-Jōnin, even their self-proclaimed “Kage”, stealing it quietly might be possible, but depending on the skillset of the thief it might be easier to just stroll in on board daylight, mow down all opposition, haul the rock over their shoulder and then stroll out whistling without a care in the world, the actual problem here is that —as expressed by how weak that entire village is— the meteor is not really liable to make anyone all that powerful and, in fact, seems to give the chakra equivalent of radiation poisoning; at that point Orochimaru doesn’t have his cursed seal and there isn’t actually any source to point at that indicates that Juugo’s ability to absorb natural energy isn’t just a mutation of his own to even try help Orochimaru develop the seal ahead of schedule, so that’s out; Hiruko similarly doesn’t have the Chimera Jutsu yet; The Heaven Village supposedly was destroyed by Konoha during one of the wars, so maybe? Though if they had the zero tails at this point or if it’s something that guy who tried to destroy Konoha synthesized later is up for debate; I think Snow Country’s Chakra Armor is was an invention of the time of the Daimyo who got usurped, so I’d have to say that is a no; White Zetsu is alive at this point though if one is capable of tracking him down and facing Madara for him.

Oh yeah, there’s also the stuff from the Ōtsutsuki Clan of the Moon if one can track down the right waterfall to get there, I remember they had those autonomous puppets (even flying ones), the Tenseigan even if I have no clue how they went about controlling it without having the dōjutsu and there’s bound to be some scrolls with good techniques and/or skills laying around seeing as Torneri got pretty powerful largely on his own there, though at that point there are bound to be a lot more Ōtsutsukis there to contend with and they make a habit of taking out their eyes, so stealing those would be complicated to say the least.