This is not Open Source but it is a good source of data to build your own software.
Component to get Latitude & Longitude from street address
Map Suite Geocode USA
Below are two other sources of data to get Latitude & Longitude. The Zip Code one will not be as accurate for your purposes as the street address one. A company that I used to be associated with in the late 1990s used this street address data for a similar purpose.
Raw Data to allow you to convert Zip Code to Latitude & Longitude
Convert street address to Latitude & Longitude (Component with local database or cloud web service)
Others available at Catalog.CodeProject.com
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