Regulated AC power supply?
The classic answers were ferroresonant regulators and motor/generator sets.  Nowadays, a good sinewave generator and a class D audio amplifier into a stepup transformer will manage the same thing, at limited power output.

At the high power end, this is often done with magnetic amplifiers, but those
aren't usually available as off-the-shelf building blocks.  Big battery chargers are
a typical application mag amp batt charger.