Not 100% sure what you mean bu get a reference to...
SystemLocationDataSource is a device dependent implementation of LocationDataSource, so having the class available in the .net shared Xamarin library is not be possible.
However, in my shared project the property mapView.LocationDisplay.DataSource will be the SystemLocationDataSource for the currently running platform
We should make that easier I agree. Currently that API is only available for platform-specific code, and not part of the .NET Standard API surface.
For now you can do what I did here:
This should return a value when running on Xamarin.iOS, Xamarin.Forms, UWP, .NET Core and .NET Framework (even if you compiled for .NET Standard).
FYI once update 9 ships, from a .NET Standard library you'll be able to call: LocationDataSource.CreateDefault() to get a location-datasource for the platform. Until then you can use the code snippet I linked to above.