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.
Guts is purely fictional. It's a concept or character that exists within the realm of imagination and creative storytelling rather than being based on reality.