Refine your "search" settings on fare comparison websites or your favorite airline's website to a departure time of "Night" or "After 9 pm," which will bias the results towards red eye flights.
Call your travel agent to ask about red eye flights that might be available.
Review an airline's timetable to see all of that airline's daily scheduled flights to and from a destination. Look specifically for the flight number of the red eye, so you'll be able to ask for it directly when consulting a ticket agent.