If you are running Windows, download and install Adobe Digital Editions 2.0.1. Install it and register with Adobe. Then you can register ADE with your Adobe account UN/PW and you can purchase ePub eBooks (not from Apple or B&N).
https://www.adobe.com/support/digita...downloads.html
The ePub that comes with DRM will need the DRM removed. You do that using DeDRM Plugin 7.0.3 (current version) and the current version of Calibre 5.x. You can get the DeDRM plugin from
https://apprenticealf.wordpress.com.
If you need help with getting the DeDRM plugin to work, you'll have to ask where you downloaded the plugin.
Once you have the ePub without DRM in Calibre, you can use Calibre to convert the ePub to KF8 (AZW3) to read on your Kindle. You can side load the converted KF8 eBook to your Kindle using Calibre.