Well, in some fanfictions, he might inherit it from an unknown ninja clan. Maybe his parents left it for him and it was hidden until he discovers his true heritage.
In certain fanfic concepts, the sword might choose Naruto. There could be some sort of magical or chakra - based connection. The sword senses Naruto's strong will and his potential for great things, so it appears before him, waiting for him to pick it up and start his new journey with it.
Well, in some fanfictions, Naruto might obtain the overpowered sword through a mysterious inheritance. Maybe it was left by a powerful ninja from the past and Naruto discovers it in a hidden cave or something.
Well, in some fanfictions, Naruto might obtain the Master Sword through a strange portal that connects his world to the world where the Master Sword exists. Maybe he's on a mission and stumbles upon it by accident.
In some fanfictions, Naruto might obtain the dragon sword through a special quest. Maybe he stumbles upon an ancient temple during one of his missions where the sword is hidden as a powerful artifact, guarded by some kind of magic or traps. He overcomes these obstacles using his ninja skills and determination, and thus gets the dragon sword.
Well, in some fanfictions, Naruto might obtain the Sword of Tengu through a quest. Maybe he has to help a spirit or an ancient being in return for the sword.
Well, he might obtain it in a fight. Maybe after defeating an enemy who had stolen Zabuza's sword from its original place. Or perhaps he discovers it in an old, forgotten hideout where Zabuza once stashed his things.
In some fanfictions, Naruto might obtain Cloud's sword through a series of adventures. Maybe he helps the Cloud Village in a great battle and as a reward, he is given the sword. Or perhaps he discovers it during an exploration of an ancient ruin that has a connection to the Cloud Village's history.
In many fanfictions, Naruto might obtain the Uzumaki sword through a hidden inheritance. Maybe it was left behind by an ancient Uzumaki clan member and Naruto discovers it in a secret location, like an old, forgotten Uzumaki shrine.
He could discover it during a dangerous mission in an ancient ruin. Maybe he was exploring an old ninja stronghold and came across the sword hidden away. It might have been protected by traps and guardians, which he had to overcome to obtain it.
Another possibility is that Naruto gets the materials as a reward. He could complete a really difficult mission for a blacksmith or a village elder. In return, they give him the unique and strong materials required to make the buster sword. He would then use his own creativity and some basic forging knowledge to shape it into the buster sword.
He could find it in an ancient, hidden temple during one of his missions. Maybe he was exploring a long - forgotten ruin and stumbled upon the sword, completely by accident.