I fitted Cobras from Low Cost Car Audio to my RAV4 (both front and back) after having a bad experience with some cheap ones.
The Cobra's have been brilliant!!
There's even a wire you cut to set the distance you want them to tone out from an object so you can custom set them to your own needs. On my 4X4 this meant that they'd take the wheel cover into account.
Basically you get someone to watch you back up to a wall, then adjust the car's position until the safety gap is to your liking then cut the looped wire and they're set for ever (unless you choose to reset them).
Maplins, 4 sensor kit with display. About £20 and works fine. I didn't want a visual display so it's under the drivers seat, which allows me to hear the beeps.
To be honest fitting it yourself is not too hard as long as you have a reasonable tool kit.
Can't really suggest anyone / anywhere to fit it for you. I do remember someone on the main reversing sensor thread saying he would help people local to him but I don't know the members name or location, sorry!