If you are a Prime member, and don't have Audible or Kindle Unlimited, well both are available for free 3 months trials at the moment.
You can't have been on a trial within the last 30 days I think.
I've signed up to both and have bagged Geddy Lee''s My Effin' Life as my first audio book.
I tend to buy audiobooks as one-offs, usually available as an addon to a Kindle title.
I've found this useful: https://getlibation.com/
.. allows you to download your audiobooks (m4b format) and convert them to mp3 (although I use a separate tool for that latter task).