Honestly, I'd say AO3 is the absolute powerhouse for that specific pairing. The tagging system over there is basically a lifesaver when you're hunting down Funtime Foxy and Funtime Freddy content. You can filter by relationship, sort by kudos or word count, and even exclude stuff you don't want. I've found some truly insane slow-burn AUs there, like one where they're rival mechanics in a dystopian city—it sounds weird, but it totally works for their dynamic. The writing quality tends to be higher, too, since the culture encourages more developed plots and character studies compared to places where it's just quick, fluffy one-shots.
That said, Wattpad has a different vibe entirely. It's way more chaotic, but you can sometimes find real gems buried under a mountain of... less polished work. The search function is a nightmare, though. I just browse the 'FNAF' tag and scroll forever. The appeal there is the immediacy; stories often feel like they're written in the moment by someone super excited about the idea, which has its own charm. You get more crackfics and silly domestic scenarios, which can be a fun break from the heavier, lore-intensive stuff on AO3.
FF.net? I wouldn't bother. The FNAF section feels kinda dead, and the tagging is prehistoric. You might stumble on an old, complete multi-chapter from like 2015, but it's a real crapshoot. For Funtime Foxy/Freddy, you really want a platform that embraces the weird ship tags, and FF.net just isn't built for that. My bookmark folder is 90% AO3, 10% Wattpad curiosities.