Any Sorey x Mikleo?
It's practically canon, but not shown or confirmed, so it's in that no man's land between so far fetched people draw porn for lulz/their headcanon and legit enough for people to expand on it.
I don't think there's really a good way? Some popular serieses have porn tags, but Tales isn't one of them. There's so much shit on Pixiv that I've gotten lazy about saving it, though, so it's not like there's a drought of porn. By the way OP, mind this being a general Tales thread? I don't mind it being Zestiria specifically, but it'd be nice.