+1. I have two Boox devices and they're both excellent. One is a 10 inch reader that's quite thin, and the other is essentially the equivalent of an e-ink phone. Where to buy books is still a challenge for me, though. So far I've been relying on my decrypted library that I've been buying over the years, but now that Amazon closed that hole, I'm no longer buying from them.
I don't have a good answer for you, I'm afraid. I'm gonna be looking into alternative stores that still allow you to download the files. I've never worked with Kobo before, but I heard that that could work. Google Play can work. And there's always Anna's archive, that's kind of a last resort for me. I prefer to pay for high-quality copies than get some scanned low-quality version, but publishers don't want to sell me a file anymore.