It depends on where you are. In the US and UK, almost everything on the Black Library site should also be available on Kindle.

https://www.amazon.com/Flight-Eisenstein-Horus-Heresy-Book-ebook/dp/B01N75PTCX/ https://www.amazon.co.uk/Flight-Eisenstein-Horus-Heresy/dp/1849708037/

I don't know which other countries also have them, but there's always the Black Library website. You can e-mail mobi files to your Kindle's e-mail address, and you'll be good to go.

Incidentally, if you're using DRM-free ebooks, I really recommend the Calibre software management and conversion.