Is there a cheap option for reversing sensors?? wireless or 9v battery or some sort of stick on the bumper type?

I bought a set of Cobra sensors several years ago.

They do require drilling into the rear vallance but provide a factory fit appearance...I also fitted an inline switch into the wiring so they can be muted if required.I find them very usefull although there is no substitute for the thing fitted to the windscreen and either sides of the car...mirrors.