Perhaps he stumbles upon it during one of his adventures outside of the village. It could be in an ancient ruin or something.
He might obtain it through a special encounter. For instance, he could stumble upon an ancient dragon's lair where the chakra is stored and it bonds with him.
He could obtain it through a powerful curse. For instance, a long - forgotten enemy places a curse on Naruto that not only gives him the blue Rinnegan but also sets him on a path to either destroy the world or save it depending on how he uses the power.
In some fanfictions, Naruto might obtain the Zero Tails through a mysterious encounter. For example, he could stumble upon an ancient artifact that houses the Zero Tails' power and it somehow bonds with him.
He could obtain it through a unique chakra infusion. Maybe during a battle with a powerful enemy who has some connection to an ancient chakra source, Naruto is accidentally exposed to a golden - hued chakra that fuses with his own and transforms his eyes into the Golden Rinnegan.
It could be that in the fanfiction, Naruto is exposed to a special type of chakra or energy source. This energy source could be related to an ancient artifact or a long - lost power. As Naruto has always been at the center of extraordinary events, his encounter with this special chakra could be the catalyst for him to obtain the True Rinnegan. It might also be that his previous battles and experiences have slowly been building up to this moment, and this final push of some external or internal factor causes the True Rinnegan to emerge.
Perhaps Naruto inherits the Rainbow Chakra. It could be a long - lost power that was once held by an ancestor. Through a series of events, like uncovering a hidden family heirloom or a secret scroll, he discovers his connection to this special chakra and learns how to unlock and use it. This would add a whole new layer to his character, as he now has a responsibility to uphold the legacy of this powerful chakra.
He could obtain it by chance. Maybe during one of his adventures, he stumbles upon an ancient temple and the Dragon Blade is just there, waiting for him to pick it up.
Another way is that Samehada might be given to Naruto by a mysterious figure. This figure recognizes Naruto's potential and believes that he can handle the power of Samehada. This then sets off a chain of events where Naruto has to learn about and master Samehada's abilities.
Perhaps he could obtain the Rinnegan through some sort of ancient ritual or a gift from a powerful entity. And to become Hokage, he would need to prove himself worthy through his deeds and leadership skills, like protecting the village from a major threat.
Another way could be through inheritance. Maybe it was his father's or some distant relative's weapon. However, it was sealed away for safety reasons. As Naruto grows stronger and faces greater threats, the weapon reveals itself to him. It's as if the weapon senses that Naruto is the right person to wield it at that crucial moment.