Potentiometers with switches used to be very common, but it's been a long time since audio equipment was made with knobs that directly control the volume.
There was a time when the on-off switch on most radios would be operated by turning the knob all the way to one end, but as soon as you want to have remote control, or anything that turns the radio on or off automatically, there can't be a switch like that on the volume knob.
I really can't think of anything less than about 30 years old that uses one of those, so it's going to be hard to find a spare part.
Can you fit a simple potentiometer and wire up a separate switch?