Perhaps he discovers an ancient spell that can summon cars. He finds a hidden book in the Hogwarts library that details this spell. When he casts it, cars from different places and times appear. Some might be from the Muggle world that have been enchanted to fit into the wizarding world, and he adds them to his collection.
He might inherit some of the cars. For example, he could inherit a classic wizarding car from his parents' old friends. Another way could be that he wins them in magical contests. There could be a Triwizard Tournament - like event where the prize is a unique and powerful car.
In fanfiction, Harry could build his cars with the help of the Weasley twins. They are known for their creativity and love for magical inventions. They could use their skills to create custom - made cars for Harry. He could also trade some of his rare magical items for cars from other wizards who are into car collecting.
Perhaps it is given to him by a mysterious figure. This figure could recognize Harry's potential and believe that he is the right person to wield the trident. The figure might be from a different magical culture that has a connection to the trident and sees Harry as a key to some greater purpose.
In some fanfictions, he might find it in the Forbidden Forest during one of his adventures. Maybe while exploring the deeper parts of the forest that are off - limits to most students, he stumbles upon a young pegasus in need of help. Harry, being the kind - hearted person he is, bonds with the pegasus and it becomes his familiar.
He could get them from his Muggle relatives. Since he spends time with the Dursleys, there might be a situation where he stumbles upon some of their tools or weapons, like a gardening hoe that he could potentially use as a weapon in a fight. Another way is through his Muggle - born friends at Hogwarts. They might bring in some items from the Muggle world that Harry then takes an interest in and decides to use.
He might find it in a hidden chamber in Hogwarts. There could be a series of magical puzzles and traps that he has to overcome to get to it.
In this fanfiction, Harry Potter could obtain Gungnir in a rather unexpected way. For example, due to a time - traveling incident. He might accidentally travel to a different time and place where Gungnir exists. There, he could be in a situation where he has to help the local magical community. As a result of his heroic deeds, the keepers of Gungnir decide to entrust him with it. And when he returns to his own time, he brings Gungnir with him, ready to use it in his fight against the dark forces.
Maybe through some kind of magical encounter in the Forbidden Forest. He could be on a special mission or adventure and stumble upon the Nemean Lion. The lion might sense that Harry has a pure heart and a great need for a powerful familiar, so it decides to bond with him.
He might find it in a hidden chamber in Hogwarts. Maybe it was left there by a powerful wizard from long ago who had some connection to other universes.
Maybe it was given to him by a mysterious stranger. This stranger could be someone who has been observing Harry from afar and believes that he is the only one who can handle the power of the hand cannon. The stranger appears out of nowhere, hands it to Harry, and then disappears again without a trace.
In some fanfiction concepts, he might change his soul through powerful magic spells. Maybe he discovers an ancient spell in a hidden book in the Room of Requirement that has the power to transform his very essence, his soul.
He might be very sad at first. Harry has always valued his friends, so being forgotten by them would be a huge blow.