If you're not looking to sail the high seas, I highly recommend Libro.fm. They have had almost any audiobook I have ever wanted (e.g., Andy Weir's books in English are only available on Audible), and you can choose a local library to support with your purchase.

I subscribe to their monthly plan, $14.99/month, which gets me 1 credit per month. If you buy one audiobook a month that's $14.99, using that credit, it pays for itself. I really like using my credits on expensive audiobooks, like $25 or something (I'm basically getting the book at a discount) and then I buy cheaper audiobooks using my credit card.

You can stream your purchased books with their app, but I download the files (everything is DRM-free) and move them into Audiobookshelf. Most of their older books are just ZIP downloads of MP3s, but newer books come in M4B format (one large file with chapter markers). Both work flawlessly with Audiobookshelf.

If you buy an audiobook on Audible (e.g., Andy Weir), you can download the AAX file from Audible and use a converter to convert the file to M4B (this strips the DRM and makes it work with Audiobookshelf). This is in a legal gray-area, depending on your jurisdiction.