"Old Work" Boxes
There are several kinds of “old work” boxes, sometimes called “remodel” boxes. Some are easier to find than others (but all are available online), and some are easier to install and more durable (details below). Instead of being nailed to a stud, “old work” boxes are clamped onto the drywall. Here are a few of the most common styles.
This tough fiberglass box is a favorite of many contractors although it's a bit more expensive (top). They like it because the rugged clamping system is much more secure than that of cheaper styles. These boxes aren't always available at home centers, but you can find them at electrical supply stores. Or search online for “Carlon 70108.” Choose this style if the outlet gets a lot of use.
This PVC box is the least expensive (middle) and most readily available, but it's also the flimsiest. Some pros complain that the clamping tabs aren't strong enough and the screws strip out the plastic.
This fiberglass box is a good choice. It's reasonably priced (bottom), available at many home centers and stronger than the PVC version.