A biography of a politician. Where to find a ghostwriter?

My mother spent her whole life serving the nation of the United States of America. At the age of 60, she retired herself from the position of advisor to the finance minister of the USA. She does not hail from a very privileged family, instead, she worked hard to achieve these positions. In the later part of her life, I want to write a biography of her life. Someone recommended me to hire professional bio writers agency to get professionalism and perfection. Please recommend me some good ghostwriter companies.