Honestly, searching for 'Roxy x Freddy' fanfic is an exercise in frustration most days. That pairing is everywhere now, but sifting through the chaff for something decent takes work. Archive of Our Own is, of course, the powerhouse. The tagging system is a lifesaver—filter for 'Roxy & Freddy FNaF' as the relationship, sort by kudos or hits, and you're halfway there. The sheer volume means you get a lot of short, tropey one-shots, but I've found some genuinely inventive AUs buried in there, like a noir detective AU that was shockingly good.
Don't sleep on Wattpad either, even if the tagging is messier. The algorithm tends to push the same popular authors, so once you find one you like, check their profile for reading lists. I've noticed the 'enemies to lovers' arc is dominant across platforms, which fits their dynamic perfectly. Sometimes you just have to wade through a dozen coffee shop AUs to find the one that clicks.