How can one get any screenplay they'd like of a feature film?

I've scoured online for screenplays for familiar, desired, and easily accessible movies online, but when they are found --they're usually incomplete. The screenplay will provide dialogue from the film, but not include action writings, character titles, parentheticals, or scene headings. An example would be this one for the film "Ouija":

http://www.springfieldspringfield.co.uk/movie_script.php?movie=ouija

Here's a trailer for the film:
https://www.youtube.com/watch?v=xBLmBdn2QF8

I can attempt to write in the rest myself while watching the movie the screenplay is for repeatedly, and presume what'd be written for those things mentioned above. It could be a good way of trying to figure out how to write screenplays in the future. That can be tedious though, and I'd like to have access to more complete/complete screenplays to compare to feature films that are familiar, desirable, and easily accessible for viewing. How can I get them? Ones for classic films? Current films? Favorite films? Have you ever gotten copies of screenplays for films you wanted them for? How did you get them? What did you do? How long did it take you to get them? Do you currently get them? How do you?

I'm not sure how you define "scouring online", but when I type 'screenplays' in to Google, a number of sites appear that host popular scripts. I was actually surprised to find that IMSDB wasn't one of them, as that's normally my first port of call for screenplays.

What the site you've linked to calls a screenplay is actually no more than a transcript. It's pretty useless to you.

And you're right; reading scripts is one of the best ways to learn how to write scripts.