How common are 400Hz supplies in aircraft?
In modern military and commercial aviation, aircraft power is 3-phase 115V at 400Hz (AC), and 28V DC.  This is standard just about everywhere.

AC power is 3-phase 115V AC at 400Hz (alternating current).  When the engines are spinning, this power is produced by onboard generators (alternators) feeding redundant power buses.  On the ground, this is typically sourced from a ground power unit (GPU).  During emergencies, AC power may be produced by an auxiliary power unit (APU).

DC power is provided by converting a portion of the 115V AC power to 28 volts DC for powering many of the avionics systems and charging the aircraft battery.

EDIT: My apologies, contrary to my original answer (I was thinking powering the cockpit displays/indicators mostly), a lot of navigation and pitch/attitude sensors take 26vac/400Hz input power.  Rate gyros, compass, flux valve, etc.  See, e.g. this section from a Cessna Citation.