Funny how sometimes things can stare you right in the face and you don't see it for long time.
Regular readers will know that I am as active as a novelist as I am a filmmaker, and often write about Artistic Cross Training. I've been aware since I saw Rollerball (1975) as a kid that short stories are excellent sources for feature and/or short narrative films. The disciplines coincide well, since both require tight, economic, use of story, character, actions, etc.
And I'm not the only one to think so. In doing some quick research, I ran across this excellent top ten list on the subject.
So what's been staring me in the face? SELF-PROMOTION!
I am published in an anthology called SPRING FEVERS and THE FALL: TALES FROM THE APOCALYPSE which are available on most e-book platforms. If you are a filmmaker looking for material, these collections are an excellent source.
Short stories are always a labor of love, since they never make any money. Authors great and small will be happy to talk to you about them, so don't hesitate to reach out.
If you're a filmmaker and not a film writer, I'd suggest you read as many anthologies as you can get your hands on. I hope you start with these.