Where to download documentaries in good quality?

I've been really into documentaries on Animal Planet and the Discovery channels lately. It's genuinely addictive, stuff about nature, technology, space exploration. Problem is I always catch them halfway through, you turn on the TV and it's already in the middle of the episode. You miss half the story and then you're sitting there confused about what's even going on. Figured it'd be easier to just download full documentary films and watch them whenever I want. Got a 4K TV so the picture should be incredible. Where can I download documentaries in good quality?