At Incheon Airport, you can buy bus tickets to go to Sejong City. Check out the maps below for ticket boxes (pink rounds) at the airport.
Terminal 1
You can purchase bus tickets and get information at ticketing offices next to Exit 4 and 9 in Arrival Hall Floor 1 (indoors) or offices next to Exit 4, 6, 7, 8, 11, 13, and 9C (outdoors).
Terminal 2
You can purchase bus tickets and get information at ticketing offices indoors on the 2nd Transportation Center (B1).