A rocker switch to replace the case power button? Or one to switch on and off your power supply?
For the former you'd need a momentary rocker switch to replace your existing case power button. Loads of people have replaced or scratch built their own PC case switches and surrounds but I'm not sure if they do a step by step for simple stuff like that. It's not much more than remove existing switch, cut wires, bare ends, fit heatshrink, assemble bare wires to switch terminals, solder, move heatshrink up, apply heat to shrink, fit switch to case.
Not sure about the latter as it sounds dodgy fiddling with all that heart stopping voltage and current.