Only some directors or producers write their own screenplays. how do other directors or producers obtain screenplays

Directors that don't write their own screenplay get them from screenwriters.

Explanation:

Screenwriters are in many cases the story creators of movies. Their job is to create a story, then the director chooses which one to tell on the screen and works on it. However, there could be the case in which the screenwriter is not the creator but the adaptor. So, they take a real event or someone else's story and adapt it to the screen. This is the product o harry potter movies, the lord of the rings movies, and many others. And that is why screenwriters are the correct answer.