How do I look for a replacement speaker?
Higher end headphones etc. do not use off the shelf speakers. Part of their "superior" sound quality comes from custom designed speakers you will not find available on Digikey etc.. Your best bet would be to contact the manufacture, although I suspect you will not get what you are looking for.

Without knowing the characteristics like Sensitivity, Power, Freq Resp. etc. of the existing speaker, even something close is probably going to lead to non uniform sound from the left and right ear.