Liquid Flow Meter - Plastic 1/2 NPS Threaded : ID 828 : $9.95 : Adafruit Industries, Unique & fun DIY electronics and kits
Water Flow Sensor Switch G1/2" Fluid Flow Meter Water Control DC 3.5-24V YFS201C | eBay
YF-S201 Hall Effect Water Flow Meter Sensor
3/4" NPT Hall Sensor Flow Meter - Leridian Dynamics
G1" Water Flow Sensor, Male Thread Brass Hall Effect Sensor Flow Meter 2~50L/MIN, DC 24V Powered Flowmeter Counter for Arduino, for Raspberry Pi for Reverse Osmosis: Amazon.com: Industrial & Scientific
Clear Turbine Water Flow Sensor with 3-pin JST : ID 5066 : $6.50 : Adafruit Industries, Unique & fun DIY electronics and kits
No-lead Water Flow Meter Sensor | Onset's HOBO and InTemp Data Loggers
E+E - EE772 In-Line Thermal Mass Flow Meter
i12 Flow Meter Sensor - Eccotemp
Viqua 410982R-20 Replacement Sensor | Buy Viqua Replacement Pro 20 Flow Meter Sensor at Serv-A-Pure