In my experience the cheapest plane ticket can only be found with a few hours of digging. If you do find a cheap ticket on the bargain bin search places like orbitz, expedia, or travelocity, make sure to go to the airlines website itself and see if you can get it cheaper or at the same price for the day and time you want to fly. Often times the airline will let you customize which seat you want which you can't do with until the day you fly with orbitz, etc (at least as far as I know).
I recently planned a trip to Australia and the lowest flights we found was from a little site called
www.airgorilla.com . We found tickets for $900 and something, at least $300 or $400 cheaper than anything else we saw. We have confirmed the seats with the airline (United!) so I am pretty sure they are a reliable company. I have no idea if this would work for everyone though.
Good luck!