Radiator electric fan switch source

I want to install an electric fan for my engine instead of the crank driven
fan.  Does anyone know of a specific temp sensor switch I can use?  I had
heard of the type that is applied to the outside of the outflow pipe.  It is a
non-adjustable type and turns on somewhere in the vicinity of 180 and off
somewhere around 155.  Any ideas of what and where to get it?

Tony