The first question is where are you located? If you’re in Hollywood or New York, pick up a rock, throw it as hard as you can and chances are you will hit a screenwriter.
If you’re not in Hollywood or New York the task gets a bit more difficult. Of course in the days of technology Skype, Zoom, Go to Meeting have made virtual meetings not only possible but in many cases preferable.
Let’s skip to the next question because the answer to number 1 will impact the answer to number 2.
Are you willing to spend money on a writer? If the answer is “yes” then you can hire a professional by contacting the agencies. Go to IMDB and look up a few films that you feel are similar the movie you’d like to have written. Find the writer or writers and then contact their agents.
This is the most direct, professional way of achieving your goal, but it’s also potentially very expensive.
Let’s go back to location. In New York or Los Angeles you could advertise for a writer on Craigslist or on some of the entertainment oriented sites… like Mandy.com, Production Hub, or ShowBizCentral etc.
When you get a nibble, ask to read a sample script (this method assumes that you are facil with screenplays, and while you may not be able to write one, you know a good one when you read it.) Find a writer that you feel has your sensibility, your taste, or as close as you can get, and hire that writer.
Some writers, particularly the inexperienced, may be willing to write on spec with the assurance that they will be paid when/if the project is funded.
The last line of defense would be to go to your friendly neighborhood liberal arts college, and look for a student in the Film/TV or English departments. Many students would work for credit and the promise of pay if the film get funding.
The same process can hold true if you’re outside of New York or Los Angeles, but you’ll have a more difficult time arranging meetings and keeping tabs on your writer.
Just to give you an idea, WGA minimums go from a low for a low budget non-original screenplay - including treatment - of approximately $49,000 to $100K for a higher budget non-original screenplay. Rewrites are an additional $24,000 and polish (a more minor adjustment) is almost $12,000.
That’s the floor and there are other expenses that the WGA requires - Pension, Health, and a $5000 publishing fee.
You can negotiate any pay structure you wish with a non-union writer, but you’re not likely to find the same quality or level of professionalism.
You can learn more at the Writer's Guild's website www.WGA.org.