We've reviewed the FenSens wireless parking sensors, which might be worth a look as they don't require any professional fitting or invasive drilling - but they're just sensors not a camera, and they aren't perfect: kit.honestjohn.co.uk/reviews/review-fensens-wirele.../

There are plenty of other options on Amazon and if you scroll to the bottom of the Amazon page, you can read other people's reviews - which is always worthwhile. Just bear in mind you'll probably need to get whatever you buy professionally fitted because most sensors need drilling into the car.