A "proper" ultrasonic dog whistle?

[C8PPO]

Anyone know of such a thing or where I might buy one? Those little metal things which you can buy for a few quid may be ultrasonic if you're 95 years old, but they're audible well within my hearing spectrum! I'd like an inaudible (to humans!) one so that I can do more training in the graden without annoying the neighbours.