Nevermind, piefed actually works correctly in this regard and you can just search like this:
https://piefed.blahaj.zone/communities?topic_id=0&feed_id=0&language_id=0&search=feddit.it&sort_by=title+desc

Then again, that only works for communities your home instance knows about.
If there's a community you find on a remote instance, that your home instance doesn't know about (nobody subscribed yet), then grab the remote community URL and search for that on your home instance, you should then be able to subscribe and others won't have to jump through these hoops.