Pokémon fandom search routines are so specific these days, I love it. Finding those Sylveon x Glaceon gems requires knowing where different communities huddle. For dedicated, longer-form stories with complex dynamics, I'd point you to Archive of Our Own. The tagging system is a lifesaver—you can filter for 'Sylveon & Glaceon (Pokemon)' or 'Sylveon/Glaceon' and then sort by kudos or bookmarks. You'll find a lot of tender, atmospheric pieces there that treat the pairing with a kind of mythical grace, exploring their contrasting ice and fairy energies.
If you want something a little more fast-paced and maybe even meme-adjacent, Tumblr's still got pockets of activity. The quality is super hit-or-miss, but you can stumble on amazing comic dubs or short, poignant headcanon-style fics in the tags. Just sifting through '#pokeani' or '#eeveelution' can yield some quick, satisfying reads that feel like you're sharing a joke with the author.
Honestly, I haven't found a centralized hub. It's more about the authors who happen to be into this very specific vibe, and they might cross-post between Ao3 and FanFiction.net. FF.net's search is clunky, but I've found a few older, adventure-focused fics there that treat them more as partners on a journey than a direct romance, which is a neat angle.