There is a Lotte Mart at Unseo station. Unseo is maybe 2 or 3 stops from Incheon airport with a travel time of maybe 5 minutes? It is true that the brands they may not carry Toto and/or Panasonic but they will have something similar.
What are your plans regarding luggage or lodging for the night? Are you staying at ICN or heading into town?
How to get there: Take the Airport Commuter Line located on B1 of the airport. You will have to walk a few minutes to get there. Make sure it is the Commuter line (on left side of area) as the Express line goes to Seoul station nonstop. Exit at Unseo station. You can only walk out of the station in one direction. First street you see turn left. Walk down the street and you will see Lotte Mart on your right side. Can't miss it. Big building by itself.