Another way could be that it is given to him by a mysterious sea - dwelling being. This being recognizes Percy's potential and the need for a powerful weapon in the face of an upcoming great danger. So, as a gift or a loan, Percy gets the trident.
It's possible that Percy has to earn the trident through a series of quests. He has to complete tasks for different sea deities or creatures. These tasks could involve things like rescuing a sacred sea creature, or retrieving a lost artifact from a sunken ship. After successfully finishing all the quests, he is finally awarded the trident.
He might find it during an underwater adventure. Maybe he stumbles upon an ancient temple dedicated to Poseidon and there, in the center, lies the trident. It could be a test set by Poseidon himself for Percy to prove his worthiness to wield such a powerful weapon.
In some fanfictions, Percy might obtain the trident through a special quest. Maybe he has to prove himself worthy to an ancient sea deity who then bestows the trident upon him as a symbol of power and trust.
One possibility is that Percy uses the trident to explore uncharted areas of the ocean. He discovers ancient underwater ruins that are protected by powerful magic. With the trident, he can navigate through dangerous barriers and uncover secrets about his own heritage and the history of the sea gods.
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.
Well, in some fanfictions, Percy might obtain the spear as a gift from a minor god who sees his potential. For example, a god of an ancient and forgotten war cult could give it to him, seeing Percy as a worthy warrior to carry on their legacy.
In some fanfictions, he might find it in an ancient weapon stash hidden in a mythical location. Maybe while on a quest, he stumbles upon a secret chamber where the katana is kept as a powerful relic from an old civilization.
It might make Percy more confident. With the trident, he has a powerful symbol of his father's support. He could use it to face stronger enemies and gain more respect in the demigod world.
He could have underwater adventures. The trident might give him the ability to communicate with sea creatures and explore the depths of the ocean. He could discover lost underwater cities or battle against sea monsters.
If Percy Jackson gets a trident in fanfiction, his relationships would shift significantly. His friends like Annabeth would see him as an even greater asset in their quests. However, some might also be worried that the power of the trident could go to his head. His enemies, especially those from the sea - related villains, would be more cautious around him. And he might attract new allies who want to be on the side of someone with such a powerful weapon.
Well, he might find it in some mysterious place like an ancient temple. Maybe it was left there by a powerful being from another dimension.
Perhaps he discovers it in an ancient, hidden temple dedicated to a forgotten god. The sword could be a gift from this god, who sees potential in Percy to use it for good.