You can start by asking for referrals from friends or family. They might know a good writer. Another option is to look online on freelance writing platforms where many writers advertise their services.
One way is to contact local writing groups or associations. They can often recommend experienced writers who are interested in writing life stories. You could also check out the works of published memoir writers and see if any of them would be interested in taking on your project.
First, decide what kind of writer you need. If you want someone with a journalistic background, look for journalists. Then, post a job description on relevant job boards or social media groups dedicated to writing. You can also attend writing workshops or events where you can meet potential writers and discuss your project face - to - face.