Go on, give us a clue: where in the world do you want the data for?
For UK routes (including UK-to-international airport pairs), the CAA has excellent data (
www.caa.co.uk).
I believe there is also publicly-available data for US-originating city pairs but I'm not sure of the source (BTS?).
IATA may have some relevant data but I don't recall if they provide public data down to city-pair level.