Hi Jeff,
Thanks for contacting Syncfusion support.
For achieving this, NuGet.config file with the required package source should be placed at the root folder of your application in the repository as per the below screenshots.
You can use the attached the Nuget.Config file from the following location.
Once you have this Nuget.config file at the root folder of your application, you can provide path to this NuGet.config file while configuring the build under Feeds in my NuGet.config.
Please check and let us know if you are still facing any issues in this.
Regards,
Saravana Pandian M