Final answer:
The navigation source for the Integrated Standby Instrument is VHF NAV 1.
Step-by-step explanation:
The navigation source for the Integrated Standby Instrument (ISI) is the VHF NAV 1. The VHF NAV 1 is a radio navigation aid that provides the necessary navigation signals for the Instrument Landing System (ILS) to function properly. It broadcasts radio signals that are received by the aircraft's ILS receiver, enabling it to accurately determine the aircraft's position and guide it during instrument approach and landing.