If you're asking for *legal* downloads, you're out of luck. You can't download HD movies legally because the whole concept of HD was to clamp down on piracy. There are a few content streaming services that will sell you ancient movies and four year old TV series, but no worthwhile content. Note streaming, not downloading.
So if you want HD movies, you have to buy BD discs and a player.
So there is now only one way for willing paying customers to watch HD online, and that way is illegal, unfortunately, and I'm not going to talk about it. I don't understand the content industry myself, but there it is.