These days when I look for a specific eBook I'll often check the author's and publisher's websites first, and see if it's available from them directly. Or if they have a preferred source.
Here are some places I've bought, borrowed, or otherwise legally acquired e-books from: https://kvibber.com/reviews/books/finding-ebooks/
As for hardware alternatives to Kindle, I like Kobo and Boox.
The Kobo Clara is the best single-store eReader I've used, and it's also able to connect to Libby/Overdrive (for library borrowing) and Instapaper.
Boox makes e-ink Android devices, so you can install any reader app you want on them (plus other apps like Wallabag, or a feed reader, or even use it for web or email if you want to). I used a Poke3 for almost 5 years before upgrading to a Go 7 Color II last fall.
@kelson Regarding reversing buttons. Check for Settings -> Smart Button Settings for "Preferred Button Function".
You should be able to choose the preferred direction (Mode 1 or 2).
Kelson
Soh Kam Yung