Can anyone tell me where I can get apk files so I can sideload please?
Why not using the play store?
Not really a Kodi question, but
http://apkmirror.com
As for the reason why normal Google Play can't be used, that's because certain apps aren't marked as Android TV compatible. Some "normal" Play apps still work with the remote, or are usable with a mouse/air mouse. Such as a decent web browser.