In my case from different places like itch.io , smashwords.com, gumroad.com, some fundraisings that provide epubs. You also have some nice free places like standardebooks.org. Or you can sail the digital seven seas if there's no availability.

Theres a long trail of smaller stores, including Kobo store and ebooks.com

Standard Ebooks for high quality public domain books, though they also offer azw.

https://www.gutenberg.org/ has more books than you will be able to read in your lifetime for no cost. Some are classics for a reason, some are garbage - same as any other publisher.

There are a number of other free ebook sites out there as well.

Anna's Archive

I wouldn't assume that purchases are taking place.