How can I trace wires from a ceiling box to find the power source?
I recently had to trace every wire in my house to figure out what the heck was going on with my kitchen and dining room circuits.

For the purpose I ended up buying Sperry's Wire Tracker. I think I paid about $50 for it. It has a base unit and a pen. The base unit has two alligator clips that you clip on to a wire you are tracing. It works best if you are tracing two wires that go to the same place for reasons I will detail in a second, but you can clip just one clip to one wire if necessary. Turn off the power, obviously.

Turn on the base unit, then hold down the button on the pen. The base unit uses the wires as an antenna for a radio signal which the pen receives. It will beep if the pen is close to a wire in the same circuit, though the signal attenuates strongly at each junction. It is strong enough to penetrate drywall, but I had the best luck by pointing it at various fictures and switches. You will get a lot of false positives, though, as other wires will resonate with the signal. When you have a candidate wire, touch the metal tip of the pen to the bare wnd of the wire. If it really is on the circuit, the beeping will get louder and clearer. If it's not, it will get staticky.

Remember where I said it's best if you can clip two wires? If you clip, say, the hot and neutral, and then you are at the next branch on that circuit so that you are looking at the other end of both wires, when you touch the two wires together the signal will stop entirely. This is a great positive affirmation that you have the right wires.

Edit: Here's an Amazon link. About $40.