Reddit was my gateway drug for this stuff, honestly. I got hooked on a 'Draco Malfoy in The Hunger Games' story ages ago, and I've been chasing that high ever since. It's less about finding one perfect site and more about developing a circuit. Tumblr is weirdly good for crossover drabbles and prompts, especially if you're into aesthetic moodboards that inspire specific AU vibes, like a 'Marauder's Era Draco' or 'Slytherins in Middle-earth' thing. The tagging system is a chaotic mess, but if you follow a few big-name blogs that reblog from smaller writers, you'll fall down a rabbit hole pretty fast.
AO3 is obviously the powerhouse, but the trick is in the search filters. Don't just search 'Draco Malfoy'. Go to the character tag, use the 'Other tags to include' field, and type 'Crossover'. Then sort by kudos or bookmarks from the last few years. You'll filter out a lot of the ancient, abandoned Geocities-era fics. Also, check the collections people have made—some users curate 'Wizarding World Crossovers' or 'Slytherin-centric AUs' that bundle 'Harry Potter' with other fandoms. I found a fantastic 'Draco in 'The Magicians'' series that way.
My contrarian take? Sometimes the best crossovers aren't tagged as such. I stumbled on a 'Good Omens'/'Harry Potter' fusion where Draco was basically Crowley's estranged godson, and it wasn't in the crossover category because the author considered it a 'fusion'. You gotta read summaries with a squint. If it says 'Draco Malfoy, Vampire Hunter' or 'Slytherin House aboard the USS Enterprise', you're probably golden, even if the metadata is a bit scuffed. The real treasure is in those weird, hyper-specific premises that only make sense at 2 AM.