Buy AppleTV's and and use iTunes, that's the only remotely consumer friendly way.
The second easiest option is Google Play Movies and a Chromecast dongle.
You can't use Plex to stream DRM media and all services use DRM infested solutions, the so called cross platform movie industry way is through a system called Ultraviolet and that method is a total mess.
P.S. you cannot purchase Blu-ray downloads, only compressed HD copies.