Head Phone Transfer Function MATLAB
It's actually a band-pass characteristic, since very low frequencies will be inaudible on headphones. Firstly, searching for "headphone frequency response" brings up a lot of results, especially on an image search. Then, the site headphone.com seems to offer frequency characteristic plots for many of the headphones they sell.

Unfortunately, you often only get graphs. However, you can use tools like g3data (plot digitizing tools) to assist you in recovering data from pictures or even scans of plots. I used such tools a lot to trace plots from scientific papers for which data is usually not available.