With #NeuroscienceMigration happening, two resources I've found:
(1) twitodon.com: register with them to link your twitter and mastodon accounts. Then it will find (and output an importable .csv of) the mastodon addresses of all of your twitter follows that have registered with them. The more of us that register, the better it works.
(2) https://fedifinder.glitch.me/: put your mastodon address in your twitter bio. Then fedifinder will find all your twitter follows (or followers, or lists) who have put their mastodon address in their twitter bio, and output a .csv of them as above.
(To import the csv: edit profile -> import and export -> import, and be sure that 'merge' is clicked.)