A certification body is an independent third-party organization that will audit your management system for the purposes of “certifying” that your QMS meets all of the requirements of a standard, such as AS9100 (however, any management system can be certified). Certification bodies differ around the world, and are accredited by what is called an accreditation body who will ensure that they are following appropriate management system audit processes.
AS I can’t really tell you for your location in the world who to choose, I would suggest searching what your national accreditation body is, then on their website they will have a listing of the certification bodies that they approve. From here you can determine which ones can certify for AS9100. Then, it is best to interview several certification bodies to find one that is best suited for your particular organization and type of aerospace industry.