Hi Joel, I just finished building one. I got my parts from Sterling Electronics in Edmonton. The part number of the Relay is ALA984H and was $5.40. The fellow working there was very helpful showing me how to connect things. The one I built has 12V running to the float switch which triggers the relay to turn on the 120V power head. Here is a schematic. If you take the schematic to Sterling, he will set you up with everything you need for the electronics.
I also added a 120V switch to over ride the power head.
Dave