Go to http://www.expedia.com/ and enter travel dates and times and click on search for flights. Usually, a long list of flights will appear. Click on the Preview Seat Availability link that appears just under your desired flight. A seat map will be shown with all available seats.

The most accurate way to see available seats is through the airline's "Seat Map" during the booking process or in the "Manage Booking" section after you have purchased a ticket. Most airlines display a real-time grid of the cabin, where occupied seats are grayed out and available seats are highlighted (often with a price tag if they are "Preferred" or "Extra Legroom" seats). However, be aware that many airlines "block" certain seats (like the front rows or exit rows) for their elite-tier frequent flyers or for airport-control on the day of departure, so the map you see might not represent every single empty chair. For a more "insider" view, you can use SeatGuru or AeroLOPA, which provide detailed maps of specific aircraft types, showing which seats have limited recline, misaligned windows, or extra under-seat storage. If you are looking for a seat on a flight that appears full, apps like ExpertFlyer can send you an alert if a specific seat or cabin class becomes available due to a cancellation or upgrade.