You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfics, and you can easily search for 'itachi x shisui' to find relevant stories.
Fanfiction.net is also a good place. There are many Naruto - related fanfics there, including ones about Itachi and Shisui. Just use the search bar and type in 'itachi x shisui' to see what pops up.
One idea could be a story where Sakura, as a skilled medic, is tasked with treating Itachi and Shisui during a difficult mission. She discovers their complex pasts and bonds with them over time, leading to new alliances and friendships.
To write a good fanfic, focus on character development. Let Sakura grow in her abilities and relationships. Show how Itachi's past affects his present actions when interacting with Sakura and Shisui. For Shisui, explore his loyalty and how that impacts the group dynamic. Also, use vivid descriptions of the Naruto world, like the Hidden Leaf Village or the battle scenes. This will make the fanfic more immersive.
One common theme is loyalty. Given their status as Uchiha, loyalty to the clan and to each other is often explored. For example, in many fanfics, they will go to great lengths to protect one another because of their loyalty.
Sakura would likely be in awe of Itachi and Shisui's skills at first. Itachi, being the more reserved one, might be a bit distant but would gradually warm up to Sakura as he sees her determination. Shisui, on the other hand, would be more friendly and approachable. He might be the one to initiate conversations and break the ice between the three. They could have exchanges about their different ninja backgrounds.
Well, one possibility could be that they are on a mission together. Sakura's medical skills could come in handy when Itachi or Shisui get injured. And they might face a powerful enemy that requires all of their skills combined to defeat. Maybe Itachi's Sharingan and Shisui's unique abilities, along with Sakura's strength and intelligence, make for an interesting dynamic.
One possible plot could be a story where Sakura, Itachi, and Shisui are all part of the ANBU. Itachi and Shisui could be mentoring Sakura in the ways of the ANBU. They might go on a dangerous mission together, facing powerful enemies. Shisui could use his unique abilities like Kotoamatsukami to gain an advantage, while Itachi's Sharingan and combat skills are also crucial. Sakura, with her medical ninjutsu and growing combat skills, could prove to be a valuable asset in the team.
Another one is 'Itachi and Shisui: The Untold Tales'. This story weaves in some unique scenarios that might not have been explored in the original series. It shows their interactions with other characters from a different perspective, highlighting their strong bond.
One popular Shisui x Itachi fanfic is 'Bond of the Uchiha'. It delves deep into their relationship, exploring their shared experiences as Uchiha clan members. The story is filled with emotional moments and great character development.
One popular 'itachi x shisui fanfic' is 'The Bond Beyond Time'. It delves deep into their friendship and how it could have developed into something more. It has great characterizations and an engaging plot.
Sakura would likely develop her combat skills further. In the ANBU environment, she would face more dangerous opponents, which would force her to become more strategic. Itachi, already a complex character, might show more of his internal struggle between his loyalty to the village and his love for his family. Shisui could develop in terms of his leadership skills, as he and Itachi might lead Sakura in missions. He could also become more jaded as he faces the darker side of the ninja world.