Archive of Our Own has the deepest archive for pretty much any Harry Potter pairing, and Fred/George is no exception. I’ve read some truly creative takes there that explore their dynamic beyond just jokes and pranks—stories that dig into the co-dependency, the shared identity, and the absolute horror of losing half of yourself. The tagging system is a lifesaver for finding exactly what you want, whether you’re looking for angst, fluff, or something darker.
That said, I also check FanFiction.net out of habit. The quality can be a lot more hit-or-miss, and you have to wade through a lot of older, less polished fics, but some of the classics are still there. I found this one years ago, 'Identical,' that stayed with me—it treated their bond as something almost mystical. My bookmark list is mostly AO3 now, though.
For niche stuff, some people swear by LiveJournal archives or Dreamwidth communities dedicated to twin-centric fic, but those can be a real rabbit hole to navigate. It's often easier to find rec lists on Tumblr that link back to those places if you're after a very specific vibe.