Dear Ms. Grainger,
Thank you for posting your request on History Hub!
For records relating to births after 1938, marriages after 1949, and deaths after 1974 of American citizens in a foreign country, please contact the Department of State, Bureau of Consular Affairs. Please see Marriage Abroad - Travel.gov for more information.
We suggest that you search these websites: Family registry (koseki) | Tokyo International Communication Committee; and Getting Married in Japan as a Foreigner for information about how to obtain a copy of your marriage license from the municipal office in Tokyo, Japan where it should have been legally registered.
We hope this is helpful.