In running down the ways that people could access "The Staircase," the crime miniseries that I reviewed below, I found a website that offers free documentaries of all types. It's called Documentary Storm, and it gives you free access — say again, free access — to hundreds of documentaries in 24 different categories from Art to War.
It doesn't have everything (my first search, for "Val Lewton: Man in the Shadows," was fruitless). But the overall selection does look interesting. I'm going to check it out this very afternoon.