@anke639: you can find several suitable ones on Amazon, such as:
http://www.amazon.com/Dwyer%C2%AE-Multi-Jet-Pulsed-Output–C-03-1/dp/B00D2ME5MC
http://www.amazon.com/Water-Meter-Stainless-Steel-Output/dp/B00GM5DCCA
there are a lot of cheaper ones with 3 wires, but those typically use a Hall effect sensor which needs to be powered.