It depends on where you are. Without knowing where you live nobody can recommend a suitable retailer. But an almost friend of mine who is slightly not English had a similar problem and solved it by placing a tiny microphone inside the headset -- until someone showed him live ATC websites.
Any largish electronics store or internet retailer can probably supply the cable you need. I can suggest one near Dallas if it helps.
I don't think the duration of the flight is so important. The cables for 25 min flights are actually the same as the cables we use for long haul.