Seriously, how have I only recently discovered this?! With all the hours wasted endlessly comparing flight-comparison websites with travel agents and airline websites, I only wish I had known about this sooner.
Google’s ITA Software provides a Flight Matrix, which allows you to search for the cheapest available flights that fit your exact specifications. It doesn’t show you where you can purchase these flights, but it gives you a great bench mark as to which price is the “true” cost of the flights you’re looking for. And it doesn’t take long from there to find the website that those flights are on when you can quickly eliminate any price that looks a bit too much.
Give it a go now; it’s worked brilliantly for me so far!