JC Whitney has a pretty cheap one for about $40.  Part number is
ZX529036B.  Has two sensors.  Dont know if it is good enough for
what you want.

I found a number of them on a previous net search.

I believe we call them obstacle detection or object detection.  Maybe
you can get better search hits with that term.

Just do a google search on backup sensors.

You know, Honey, you could just install some kind of bump on the
pavement, exactly at the spot where you want your wheels to be. Then
back up slowly until you feel the rear wheels hit that bump!

Someone could move it back on you as a practical joke, while you're
gone, so you hit the wall first. For that, you need a bump-move sensor
with a wireless transmitter. When you are within range of the bump, the
receiver device can pick up the signal from the bump which tells it
that it has been moved! That can be turned into a voice warning:
"warning: bump has been moved! Vehicle may contact wall!" And  of
course, if no signal is received: "Warning, bump malfunctioning,
stolen, or bump battery low. Vehicle may contact wall!"

I've also seen trucks and vans with foam padding taped to the rear
bumper. The idea there must be to keep backing up until you hit the
actual wall.