Well, it's not going to be pretty and may not be acurate, but you could use an HVAC controller. They all have switches and relays for controlling electronic equipment.
The difficult part is adapting it to the temperature sensing you'd like to do. If you felt like doing some soldering, I suppose you could pull the thermocouple inside, wire it, then shrink wrap it so it could be placed in a sump or something. You'd have to use a digital one, though, because the old slider-style ones just rely on the bi-metal type temperature sensors and a mercury switch.