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.

Not quite. If you use the "send to kindle" feature, since 2022 Amazon will convert it for you before it puts it on the device.

If you copy an epub to the device over USB it won't work. The kindle itself does not support epub.

really, epub without conversion? Which kindle can do thay?