I am looking for an system that will shut the water off if there is a leak. I have seen a few that have sensors you place at each water using device. However I found one that monitors for abnormal flow. It is pretty expensive though:
Fire suppression systems (water) in industrial or commercial buildings do have water flow vane switches in the supply loop so the the local alarm service will know there is a problem and yes, they are expensive. You should be able to get one from a plumbing supply house. They are adjustable for flow sensitivity from full on down to a cup a minute, but they won't do much for a small leak. What you should probably look for is a small dampness detector you place on the floor just below a water fixture. It will scream if it detects a dime size puddle of water. Then if you are an electro-geek you can build up the circuit to control a solenoid valve on your main water service. You would need a detector for every H and C aperture in your home ( in my case about 20 of them) and some would have to buried in the walls -bathtubs or wall showers-. You can get them usually at a local hardware store. Good Luck.