You can check to see if each individual rear sensor is working by putting the car in reverse with the ignition on. Bend down with your each next to a sensor. Touch the sensor each tap of the sensor will produce a faint but audible sound, repeat for each sensor.
I have identified that one of my sensors is not working using this above mentioned technique.
Given the car is 16 years old then it's a judgement call as to what is best for you. I don't know your diy ability. So if your into diy, I would give it a go, failing that you can get dirt cheap Chinese rear sensors kit.
As for individual sensors, knowing Honda they probably only sell you 4 sensors but that is only my guess.
Have a look on eBay they do the whole kits and individual sensors, note that some come unpainted. The ones I have came painted Black to match the car.
I would be tempted to borrow a sensor just to make sure that is the fault.
Ben