Screenwriting is the heavy lifting for your movie. Without a good script you limit your chances for success from the onset. Your goal is to only work with the best material you can get your hands on. You will want to decide if you will write your screenplay yourself, with a writing partner or if you will acquire a screenplay from someone else.
If you choose to write your screenplay yourself, do not be afraid to write a crappy first draft. Most screenwriters in Hollywood claim to have a screenplay, but that is not true. Rather most would-be screenwriters have the first 15 pages to screenplay and they will never finish. This is because they are afraid of failure. But not you. Your goal is to write, and write, and write. Finish a crappy first draft. Then refine it.
In the event you simply want to acquire good material, many filmmakers post an ad on craigslist.com whereby seeking a competent screenwriter. But if you do this I guarantee you will be inundated by gazillions of writers seeking a producer. I suggest that you research established but new screenwriters who have a similar vision. You may have to pay, but at least you will know the type of work you are getting.
For additional screenwriting resources, check out the Indie Producer’s Guide To Writing Movie Scrips.