Lemmyverse is a search engine for Lemmy (though it doesn’t find communities on Kbin (yet). It can help you find communities you’re looking for, and also shows how big they are so you know where to go for most activity.

Sub.Rehab and RedditMigration.com are indexes of communities that have at least created a counterpart somewhere other than Reddit (though they don’t track activity so some might be rather small still).