The package sources can be found in the Visual Studio options (Tools -> Options) under NuGet Package Manager -> Package Sources or directly by clicking on the according icon in the NuGet dialog (context menu of a solution/project -> Manage NuGet Packages...):
Your local package source should then be listed in the following dialog:
The package source may, however, be solution or project specific and may therefore be specified in a NuGet.config file in the solution directory. Beginning with NuGet 3.4, Visual Studio looks in the project’s directory or “or any folder up to the drive root”, according to the NuGet.config reference. Up to NuGet 3.3, also subdirectories with the name .nuget where searched for NuGet.config files.
The file containing your local package source must be changed in order to restore the correct packages.