A long copper bar inside a service panel or breaker box. All neutral and ground wires are attached to a ground bus bar to complete the electrical path. In some cases a service panel may have separate bus bars for neutral and ground.
There will probably be several options for an electrical source to serve a new outlet. For instance, it is likely there are several suitable wall receptacles in any given room to extend the circuit to another room or another floor of the house.
Also called an old-work box, these are electrical boxes with built-in clamps to fasten them into finished walls. All that is needed to install one is to cut a hole the proper size in the hollow part of a wall and insert the box. These can be used in drywall, plaster or paneling. They are best suited for adding wall receptacles and switches. Because they don’t offer much support, old work boxes are not a good choice for installing a new ceiling fixture.
A wall outlet box that houses two devices side by side as with two wall switches at a door. Triple-gang boxes are also commonly available to hold three devices in one place.