Purchasing German Marriage Records

My information on my ancestor Anna Margareta Ruhl comes from a book on the Alt family. The author is now deceased. There are copies of original documents of other records, but the information on the marriage just says Johannes Chritoph Alt married 20/11/1817 at Nieder Moos, Germany to Anna Margareta Ruhl. I am thinking I should purchase this certificate and I am wondering what the process is please?