You should not put any insulation or sealing materials inside the electrical boxes as the air space is needed to keep the wires from overheating.
In new construction we set the electrical box inside a "poly-pan" which then seals down to the vapour barrier, giving an air-tight electrical box. Now you can also buy electrical boxes without any holes in them, and with rubber gaskets where the wires go through. These usually seal to either the vapour barrier or the drywall to stop the drafts.
If you do not want to open up the wall, there used to be rubber inserts that you could put inside existing outlets, passing the wires right through the rubber. I had lost the contact for these but after we did this show I managed to find out that not enough of these sold and they were taken off the market. The molds still exist but none are being made today. So, short of putting in a fully air-tight electrical box, you can greatly reduce the air draft by using outlet gaskets under the faceplate and inserting childproof caps into the prong holes. Unless someone wants to start manufacturing the rubber boxes again.