@MessengerFlats Try this site. Not perfect but I found a few. Useful for rebuilding networks.
https://pruvisto.org/debirdify/

@MessengerFlats https://fedifinder.glitch.me/
If they have added their mastodon handle to their twitter profile this will find them.
You can then use the file to import them in the preferences -> import/export -> import
make sure 'merge' is selected