Arise (parts 1, 2, and 3) are on Netflix. I had to use Wikipedia to tell which order they were in, until I read the descriptions more closely. This is a prequel, of how the characters came together.

Stand Alone Complex is on DVD. I like it a LOT more than the movies, as it really portrays more characters as 3-dimensional than just the Major and her partner, Bateau. I don't know if it's on Amazon Video, but there's a decent chance of it.

If you know where to get them, you know as much as I do!

The only "official" way seem to be the BDs.

Hulu has a few of these