A wall receptacles with two outlets. These are available rated for 15 and 20 amps, as well as special ground fault circuit interrupters (gfci) and isolated ground receptacles.
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.
Fit a nail guard into each notch, covering the new wires. Drive the plates down tightly making sure they are flush and not protruding from the stud surface.
Use a drywall saw and knife to cut drywall and gypsum board plaster, use a reciprocating or keyhole saw to cut wood lath plaster. With the framing exposed, use a small saw to cut two vertical kerfs in the plate about an inch apart and then use a wood chisel to chip out a notch about ½inch or so deep, between the cuts.