I've got a two piece gadget that works nice for what you're talking about. The transmitter plugs into an outlet and sends a signal on that circuit. Back at the breaker box, the receiver has a wand that you can chase down the signal with. Sometimes it will only get you within a breaker or two, but when you switch off the right breaker, it kills the tone and then you're certain. The transmitter also has some adapters to connect to open wiring, say in a junction box or light circuit. Mine is a Gardner-Bender. Here's a link.