Well, in some fanfictions, he might get it from a skilled ninja prosthetics maker. This maker could be someone who specializes in creating artificial limbs for ninjas who have lost their arms in battle.
One common theme is adaptation. Naruto has to adapt to having a fake arm, whether it's in combat, training, or daily life. Another theme is often power - up. The fake arm might give him new abilities or enhance his existing ones. For example, it could be a super - strong arm made of some rare material that lets him punch through walls or something.
It could have enhanced strength. Since Naruto is already quite strong, the fake arm might be designed in the fanfiction to multiply his physical power, allowing him to punch even harder or hold heavier objects.
One interesting Naruto fake arm fanfiction could be one where Naruto gets a special prosthetic arm after a battle. It might have unique abilities, like being able to channel chakra in new ways. Maybe it was crafted by a hidden master. Another could focus on how he adjusts to having a fake arm in his daily ninja life, dealing with tasks that were once easy but now are a challenge.
You can try fanfiction.net. It has a large collection of Naruto fanfics, and there's a good chance you'll find some 6 arm Naruto fanfics there. Just use the search function and type in relevant keywords like '6 arm Naruto' and browse through the results.
One popular 6 arm Naruto fanfic might be 'The Six - Armed Ninja'. In this type of fanfic, the concept of Naruto having six arms could bring a whole new dimension to his combat abilities. For example, he could use multiple jutsus simultaneously, like performing Rasengan in two hands while using shadow clones with the other four arms for a coordinated attack. Another fanfic could explore how his friends and the village react to this strange new transformation. Does he face fear or acceptance from his peers?
In the context of the fanfic, it could also impact his relationships. His friends might be overly protective, which Naruto may not like. Or, he could inspire others with his determination to keep going despite his injury. It might also change how he is perceived in the ninja world. Some may see him as weakened, while others could view him as even more of a hero for his ability to persevere.
A possible main antagonist in such a fanfic could be a dark entity that has possessed someone close to Naruto. This entity could be using the host's body to get close to Naruto and then attacks him, resulting in the loss of his arm. Naruto would then have to figure out not only how to deal with his injury but also how to save his friend from the dark entity's control.
Perhaps in the fanfic, Naruto loses his arm due to a curse or a forbidden jutsu gone wrong. His friends, like Sasuke, then try to find a way to either get him a new arm or help him come to terms with the loss. It could also show Naruto's journey to adapt to his new disability and still be a great ninja, inspiring the younger generation in the process.