There are several options. Some online manga platforms like Crunchyroll Manga or Viz Media offer it. You could also check out local comic book stores that carry a wide range of manga titles.
I'm not entirely sure about the specific plot details as I haven't fully read it. But generally, it might focus on Sasuke's further adventures, his growth, and perhaps some new challenges he faces in the Naruto universe.
The central character is Sasuke. In addition to him, it's likely that we'll see characters such as Kakashi, who has mentored both Sasuke and Naruto. There could also be some new characters introduced specifically for the story in the novel. Given Sasuke's past associations, characters from the Uchiha clan or those related to their history might also make an appearance.
In Sasuke Retsuden fanfic, aside from Sasuke, characters like Kakashi could be main characters too. He is a respected ninja with a wealth of knowledge. His guidance or his own story arcs that intersect with Sasuke's could be part of the fanfic. Also, new characters that are created specifically for the fanfic might also emerge as main characters, depending on the direction the author takes. For example, a mysterious ninja who has a connection to Sasuke's past and becomes an ally or an enemy, which would make them a central figure in the story.
I'm not entirely sure as I haven't read it fully myself. But it might be about Sasuke's further adventures, his past experiences that haven't been fully explored before, and his relationships with other characters in the Naruto universe.
In Sasuke Retsuden fanfic, one interesting plot could be Sasuke's exploration of new jutsu. He might encounter unique challenges during his journey that force him to develop new skills, which is always exciting for fans who love to see his growth as a ninja.