Best EPUB Book stores?

I got my XTEINK X4 today. Sadly, most of my books have been purchased through Amazon, and I recently got a newer Kindle Paperwhite. So all of my books are DRM-protected, and I can't seem to strip them with Calibre.

So I'm looking for options.. lol.

Are there online stores that sell epub version? Is there a newer workaround for stripping DRM? HELP.. lol