Working through the channel, measure the distance between the joists and cut a 2x4 to this length. Attach a fan-rated outlet box to the midpoint of the 2x4 where you want the fixture to be. Attach a joist hanger to each end of the 2x4, keeping them flush, not protruding past the end of the wood. Butting the 2x4 against a flat surface while assembling can help to keep the bracket set properly. Insert the support assembly between the joists and set it so the outlet box is flush with the ceiling surface.
This page offers some options for locating an electrical source for a new wall receptacle or a light fixture, and running the new cable required. To make this job easier, it’s best to have a second person at one end or the other of a new run, to sight the end of the fish tape and help guide cable. Also be sure to always turn off the affected circuits before working on household electricity.
Locate the two joists nearest the new fixture location using a stud finder or tap on the wallboard until the sound indicates the general location of the joists. Use a drill and an eighth inch bit to drill through the plaster where the joists are estimated to be. Drill holes at 1" intervals until you meet resistance and wood from the joist is ejected. Mark the location of each joist with a pencil mark.
Where no means exists within the snap switch enclosure for connecting to the equipment grounding conductor or where the wiring method does not include or provide an equipment grounding conductor, a snap switch without a connection to an equipment grounding conductor shall be permitted for replacement purposes only.