I have regularly found cheap flights from www.momondo.com. I have also had some success in the past using http://matrix.itasoftware.com/search.htm. The latter is a flight calendar / matrix powered by Google that shows you the cheapest available flights for your selected date range (assuming you have some flexibility in booking). You can't actually purchase the tickets from the ITA site, but at least you will know what type of dollar value you should be looking for (or if all else fails, it will give you something to stand on when bartering with a travel agency!).