I spent way too much time last month trying to hunt those down, and honestly your mileage is going to vary wildly by what you mean by 'spicy'. Like, are we talking fade-to-black or full-on explicit? AO3 is hands down the archive for the latter, no contest. Their tagging and filtering system is a lifesaver—you can sort by explicit rating, choose 'Bakugou Katsuki/Reader', and filter out any tags you hate. The quality's a total grab bag though, some of it's cringe and some of the authors are genuinely fantastic at characterization.
A03 is definitely my first stop, but Tumblr has its uses. You can find dedicated blogs that recc or write them, but it's more of a scavenger hunt. The tagging is a mess and you have to know specific phrases like 'bakugou x reader smut' or 'katsuki x you' to dig anything up. Twitter's similar, but I find it even more chaotic for actually finding fics to read, better for following specific writers.
Tiktok and Instagram surprised me with their fanfic communities, especially for shorter, punchier stuff. Creators will post little spicy snippets and then link to their AO3 or Wattpad for the full story. Wattpad itself is... well, it's Wattpad. It's where a lot of new writers start, so you get a ton of 'y/n' stories, but the tagging is less precise and the search function is pretty bad. It's like panning for gold in a muddy river, but you might find a shiny piece if you're patient.