In the US, Hulu offers every single one of the movies, so it should be your one-stop shop. It's also where Alien: Earth will land. A subscription starts at $9.99 but there are sometimes Hulu deals.

MY SAY In spite of that famous source material (xenomorphs!) and pedigree (Hawley), "Alien" fans may want to temper their expectations here. Foremost, "Alien: Earth" is not the movie or even a movie, but a series full of character exposition and backstory. With eight hours to fill, the pacing can be unhurried too.

So, what Alien: Earth is saying is Weyland-Yutani is aware of xenomorphs (among other creatures) well before the events of the films and was *this close* to getting one back to Earth.