Well, a common concept is the misuse or misinterpretation of invocations. Maybe a character accidentally uses an invocation in the wrong way and causes chaos, like turning a classroom into a swamp instead of just making a small puddle of water. This can lead to humorous situations or even more serious consequences that drive the plot forward in the fanfiction.
Hermione Granger is often popular. Her intelligence and thirst for knowledge make her a great character to explore invocations with. She might be the one to discover a new invocation or figure out a complex one. Harry Potter himself is also a favorite. His courage and connection to powerful magic can lead to interesting invocation - related storylines, like using an invocation to face a particularly difficult dark wizard.
Well, you could start your search on dedicated manga websites. Some well-known ones are MangaReader or MangaPark. Sometimes, libraries might have digital or physical copies too. Also, keep an eye on official publishers' websites for availability.