In no particular order:

• booru.allthefallen.moe
• burggit.moe (use the search function)
• Some mastodon (and misskey) instances (in order)
  • baraag.net (try #animated (or its variations) or the new search feature)
  • aethy (maybe, I don’t really use the site, but they’re pretty big an CWs)
  • misskey.io (Japanese, searching might be harder)
  • pawoo.net (mostly Japanese, and there are some pedos)
• pixiv (maybe, haven’t used the site in a while)
• inkbunny.net (if you’re into cubs/furries)

If you’re only looking for animations, it ultimately comes down to how good people are at tagging stuff. I don’t know of any sites that are exclusively for long animations, but you should be able to find some on most of these sites. Can’t say which is the best with tagging, but booru.allthefallen.moe seems pretty good.