Why do people have Mastodon names in their Twitter profiles? Because https://fedifinder.glitch.me exists:
1. Open the link & log in. (No, they won't learn your credentials.)
2. If people you follow have their Mastodon handles in their profiles, this will give you a list of those. (Which includes email addresses, sadly.)
3. You can even download a CSV and import it here in Mastodon. (I haven't tried that, because I only had half a dozen handled listed.)
Thanks to @luca!