I am trying to locate a co-author for a book series I have planned. I have the basic premise and foundation done, and have written a rough "first page." But, even though I am a published author in my own right, I do not feel proficient to write in the genre the idea demands. Any ideas for how to locate such an individual?
One approach, is by sharing your concept with your agent and your publisher, they might have ideas of other authors that would be a good fit for collaborating on your story. Of course, you'll have to reach out to those authors and put the proposal to them.
Or you can reach out to other published authors via social media, assuming you don't have a professional or personal relationship already with him. I'm thinking things like doing a book-tok on an author's new release, talking about how much you enjoyed the writing. Or supporting the author's causes that you also support. Those kinds of interactions can turn into professional relationships.
The last thing that occurs to me, seems more far a field, work with online or local writing groups to meet more writers. There are lots of writing groups on meetup.com meet via zoom. Local organizations like Northern Colorado Writers put on critique and coffee clutch sessions for paying member. And there are lots of online groups like critters.org were writers can share and critique their work. Working with these communities can broaden your writing circle and might help you find effective co-authors.
What you are looking for is a ghost writer. Post a job ad in a relevant job portal.
Commonly, co-authors share writing duty. The most frequent forms of this collaboration are dividing narrative viewpoints among the authors (e.g. one writer writes one protagonist, the other writer writes the other protagonist) or the authors exchange the manuscript and take turns revising what the other has written and adding new sections to the text.
Famous authors sometimes employ younger colleagues to write from their outlines and under their supervision. For the aspiring writers this is both a chance to learn from the experienced writer and to gain exposure. James Patterson is a prime example of this strategy. As you say that you "do not feel proficient to write in the genre the idea demands" you probably cannot offer the learning that a young writer might seek, and I assume you aren't famous enough to guarantee the bestselling publication that might serve as an incentive.
Outside of such special cases where established authors offer fame and fortune, few writers will be willing to do all the hard work of writing a story that isn't their own for a dubious chance of success and having to share authorship and profit with someone who basically did nothing. Ideas are cheap and countless people who cannot write are looking for someone to write what they believe will be a bestseller from their ideas. But most writers have more ideas than they will ever be able to write in a lifetime. Finding ideas isn't the difficulty in writing, the writing is. And that is why very likely no one will write your story for you for free. But there is a large market for ghostwriting, and if you pay fairly, you can find someone with the necessary skill.
Search for "ghostwriter wanted" or something similar and see where such ads are published. Ask for samples of published work to evaluate the quality of the offers.