Six dollars - https://www.homedepot.com/p/resideo-Water-Heater-and-Sump-Pump-Single-Use-Water-Leak-Detector-with-Alarm-RWD11/204116887 https://www.homedepot.com

Six dollars - https://www.homedepot.com/p/resideo-Water-Heater-and-Sump-Pump-Single-Use-Water-Leak-Detector-with-Alarm-RWD11/204116887 https://www.homedepot.com/p/resideo-Water-Heater-and-Sump-Pu... There are a bunch of options at Home Depot, Lowes, and your local hardware store.