You can buy outlet insulators for a few dollars or you can make them out of clean, meat trays.
You take the cover off of the outlet, lay it on the tray and trace around it and around the holes. Cut it out and place it over the outlet. Replace the cover.
Any unused plugs should have safety plugs in them as cold air can travel through the plug itself. You can buy those in the baby department.
You do not need to insulate outlets on inside walls.