I have 2 of 'em.. I think I got one at a "Farm & Fleet" type place (possibly Tractor Supply center) and the other at Home Depot or Lowes.
They are sold in the farm stores for stock tank heaters
And in the Home Improvement stores for pipe heaters and eves heater strips.
One of mine looks a bit like a standard 3-way outlet adapter (the flat kind which plugs into one half of a duplex receptical or the outlet end of an extension cord and has 3 outlets side by side) but it only has 2 outlets.
The other is a bit different it has only one outlet, and an indicator light.
I use one for the tank heaters (inside the wet bay) and one for the hose heater (Free Air)