Movie/TV Scripts.  Just where do they come from and who writes them?   Well, that's not a question easily answered.  Scripts can actually come  from anywhere. In the past, scripts use to come primarily from the  Hollywood/New York markets. Now, they can basically come from  just about anywhere.  

Who writes these scripts? Use to be, scripts were written mainly by  professional writers - now, they can come from doctors, airline pilots,  car mechanics, taxi cab drivers and yes, even housewives.

No matter who you are. If you have a good story - Hollywood may be  interested in buying it. Now, suppose you have a great idea for a script, but are unfamiliar with scriptwriting. Well, the best thing for you would be  to learn scriptwriting and write your own story. Or, you could hire a freelance  writer to write the script for you. You simply supply the freelance writer with a complete outline of your story with characters and all - and they will write  the screenplay. 
 
Where can you find freelance scriptwriters at a decent price? Your best bet  would be to check out your local college theatre and film departments.  Post an  AD in the college newspaper, or post an ad on a bulletin board at the college.

Also - sometimes producers will purchase just the story idea by itself.  However,  when a producer purchases just the script idea, it's almost always purchased for  a lot less than a completed screenplay. I met a movie producer once from  Warner Bros. Studios - and I told him that I had some good ideas for a movie,  but did not want to write the screenplay. I asked him if he would be interested in  looking at some of my ideas.  He not only told me that he is always looking for  new and good material - he even handed me his business card and told me to  stay in touch.  So that shows you right there - producers do not really care where  the scripts/ideas come from ... as long as they are good - they want 'em!