WHERE TO FIND GHOSTWRITERS FOR BOOK WRITING SERVICE?

I have been looking for the options that can help me write a couple of chapters for my book, and I came to know about the ghostwriting services. I was told that even ghostwriting services can be availed in multiple forms. According to my current information now, ghostwriting services can either be gathered from the professional ghostwriting agencies or by hiring a freelance ghostwriter to do my work. Now my question is which agency should I go to, in order to get a quality service, or premium ghost writer if I pursue with my option of hiring a freelance ghostwriter? Kindly share your views.