First you can buy a few mains networking widgets with through sockets, something like the BT Broadband Extender Flex 1000 kit. Then you pull one apart, and find out what it is built from. You operate another, to see what it does. You hack a third, install linux on it, add a relay for the mains output etc. That's called research. Thus informed, you can then go to a design house with real specifications and a stab at a bill of materials.