We've tested the Fensens wireless parking sensors. It's a very smart bit of kit and, as its name suggests, has the benefit of not needing to be drilled into place on your car. Though it does admittedly work better with Android phones than iPhones. You can read the full review here: kit.honestjohn.co.uk/reviews/review-fensens-wirele.../

There are much cheaper alternatives on Amazon, but obviously you'll get what you pay for and most need installed by someone who knows what they're doing.