Perhaps Hagrid could be his mentor. Hagrid has a lot of knowledge about magical creatures and their parts, which could be useful for Harry in his blacksmithing. He could show Harry how to use things like dragon scales or unicorn hairs in his creations.
Griphook, the goblin, could be Harry's mentor. Goblins are known for their excellent metalworking skills. Griphook could teach Harry the finer points of working with precious metals like gold and silver in a magical context. He could also show Harry how to inscribe magical runes on his creations, which would enhance their power. Although goblins and wizards have a somewhat tense relationship, Griphook could see potential in Harry and decide to help him develop his blacksmithing skills.
Dumbledore could be his mentor. He has vast knowledge and experience, and he would likely see the potential in Harry being an Animagus. He could guide Harry through the difficult process of learning to control his transformation and use his Animagus form for good.
Maybe an old wizard who has been secretly studying the combination of different magical universes for years. This wizard could have made some initial attempts at building a lightsaber - like weapon in the past and can share his experiences and mistakes with Harry. He would teach Harry about the balance between the light and dark side of this new form of magic, just like the Jedi teachings in Star Wars.
It could be Hagrid. He has a lot of knowledge about different magical creatures and skills. He might introduce Harry to marksmanship as a way to protect himself better in the wild. Hagrid could show Harry how to make his own arrows and teach him the basics of aiming and shooting.
It could be an old wizard who has been living in seclusion for centuries, perfecting his smithing skills. He might emerge during a time of great need in the wizarding world, like when Voldemort was at his most powerful.
Lupin could be a very interesting mentor. He could teach an apprentice about dealing with their own inner demons, as he has his own struggles with being a werewolf. He could also pass on his knowledge of defensive magic and how to use magic in a more practical and street - smart way.
It could be an older relative who was thought to be dead but actually has been living in seclusion, waiting for the right time to teach Harry about their family magic. This relative might have a vast knowledge of the family's magical traditions and spells.
Snape is a likely candidate. He is a skilled legilimens in the original series, and his relationship with Harry is already complex. Teaching Harry legilimency could be a way for Snape to either help or hinder Harry, depending on his motives.
I'm not entirely sure if there is a specific one with that exact premise. However, fanfiction authors are constantly coming up with new and unique concepts. Given the popularity of both Harry Potter and Naruto, it's possible that someone has written a story where Harry Potter is a blacksmith within the context of a Naruto fanfiction. It could be an interesting take on the characters and their worlds, perhaps exploring how Harry's skills as a blacksmith could interact with the ninja world of Naruto.
A Muggle - born blacksmith who discovers the wizarding world and uses his Muggle skills combined with magic he learns to create unique magical items. He could be the main character, struggling to fit in the wizarding world while also making a name for himself as a great magic blacksmith.
Maybe Dumbledore in a more in - depth way. In the original series, Dumbledore was somewhat mysterious in his teachings. Here, he could really focus on Harry's growth and give him proper training, perhaps in the art of divination and understanding prophecies better.