Marriage records are county courthouse records. Many records for the time period you mentioned are available through Ancestry.com, familysearch.org, or state archives. You did not give her name or the place she lived, so I'm unable to be more specific as to where you should start looking for records. Unfortunately some of our ancestors travelled to unknown places to marry, but you should always check close to home first.
Dear Ms. Marrin-Fountain,
Thank you for posting your request on History Hub!
The Federal Government does not ordinarily create or maintain birth, death, marriage or divorce records. Such records are made and kept by State and local governments rather than the National Archives. We suggest that you search for your grandmother’s marriage in the Historical Records Collection “United States Marriages” and also review the FamilySearch research wiki for United States Marriage Records.
We hope this is helpful. Best of luck with your family research!