Kreis Archive Nienburg (2) [Weser] Germany Civil Registration Records on Ancestry.com: https://search.ancestry.com/search/db.aspx?dbid=61522. Contains many places surrounding Nienburg.
Index to births, marriages and deaths in Standesämter in Kreis Verden: http://regionalforschung-niedersachsen.de/links/
Index to births, marriages and deaths in Standesämter in Kreis Diepholz: https://www.verdener-familienforscher.de/verden/datensammlung/Datenbanken/stadiepholzster/index.php?id=main1
List of Civil registration records in various archives in the Elbe-Weser Triangle: http://www.elbe-weser-forschung.de/standesamtl-erstregister/
Overview of the civil registration docments in Stadtarchiv Leer: https://www.stadtarchiv-leer.findbuch.net/php/main.php
Civil Registration records are indexed at MyHeritage for Germany North Rhine Westphalia for deaths and marriages.