A lot of people would probably say Archive of Our Own first, and I get it—tags are so good for finding that specific farmer/Wizard or Sam/Reader mood. But the Tumblr community is where a lot of the first drafts and little headcanon snippets live. You find an artist who draws cute Sebastian with piercings, and in their reblogs there’s a link to a three-part mini-fic about teaching him how to fish. It’s more scattered, kind of a vibe-based search instead of a tag-based one.
Some fics also cross-post to Quotev or even Wattpad, though the quality can be a real mixed bag on those. I stumbled on a surprisingly solid Elliott slow-burn that was only on Quotev; the author said they liked the formatting better for reader-inserts. So my method is usually check AO3 for the polished, complete works, then tumble down the Tumblr rabbit hole for the raw, immediate ideas and art combos.