So, 'My Hero Academia' fanfiction? It's absolutely everywhere, but the quality and type vary wildly by platform. AO3 is my main haunt. The tagging system is unbeatable for finding specific dynamics, whether you're after BakuDeku angst, TodoDeku fluff, or rare poly ships involving Shouto. The writing tends to be more polished and the stories are often novel-length.
That said, I still have a soft spot for FFN for some classic, plot-heavy fics. You'll find a lot of the older, foundational works there, especially if you're into longer adventure arcs that aren't strictly ship-focused. Wattpad can be hit or miss, but the 'Bad Boy Bakugou' or 'Ice Prince Todoroki' tropes are alive and well there, which is its own kind of fun for a quick, tropey read. Tumblr is essential for headcanons, drabbles, and that immediate fandom pulse—you won't find complete epics, but you'll find the raw, emotional core of why people love these characters together.