Been scrolling through Archive of Our Own a lot lately, and that's honestly the top spot for Finn/Flame Princess these days. The tagging system is a lifesaver—you can filter for 'Finn x Flame Princess' or 'Fubble' (that's the ship name if you didn't know) and sort by kudos or hits to see what's trending. A bunch of really solid writers hang out there, doing everything from post-'Vault of Bones' fix-its to wild AUs. I'd skip FanFiction.net for this specific pairing; the tagging is a mess and it feels like the activity moved over to AO3 years ago.
Sometimes I check out Tumblr blogs dedicated to Adventure Time fanworks. They often recompile and recommend stories, so you can find some hidden gems that way. Just be ready to do a bit of digging through reblog chains.
honestly, the 'best' stuff for Finn and Flame Princess tends to be scattered. Archive of Our Own is the biggest hub now, but back in the day, a lot of great authors posted on individual Tumblr blogs that have since vanished into the ether. The trick is learning how to filter on AO3—sorting by kudos or bookmarks gets you the popular ones, but I've found some real character-study gems by sorting by 'date updated' and looking for recent, passionate works from new writers.
Your mileage will vary a lot depending on what you're looking for in their dynamic. Do you want post-canon reconciliation fics that try to salvage something from that messy breakup? There's a whole subset that's really thoughtful about Finn's maturity and FP's leadership. Or are you here for the alternate universe stuff, where they get to be happy from the start? I'm a sucker for a good fantasy AU where their elemental natures are part of the worldbuilding. Honestly, some of the best writing I've seen for them isn't even explicitly romantic; it's gen fics that just explore their unique perspectives on heroism and responsibility.
Don't sleep on crossovers, either. I once read a wild but brilliant one that dropped them into the world of 'Avatar: The Last Airbender' and treated FP like a firebending prodigy. It was more about her journey than the ship, but their interactions felt incredibly true to character. Finding that kind of thing is half the fun, even if you have to wade through a lot of less-polished stories to get there.