Here are a few examples of attenuators you can use for headphones:

1. https://www.amazon.com/Shure-EAADPT-KIT-Adapter-Combines-Attachable/dp/B001P5HL3S/ref=sr_1_2?keywords=attenuator+mp3&qid=1569248903&s=electronics&sr=1-2
2. https://www.amazon.com/Ear-Buddy-Headphone-Optimizer-Travel/dp/B075PVVHF5/ref=sr_1_2_sspa?keywords=attenuator+3.5mm&qid=1569248936&s=electronics&sr=1-2-spons&psc=1&smid=A34JWT04R7KMFW&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEyR1ZDUlNZS0JYWkhEJmVuY3J5cHRlZElkPUEwNjM4MjY1M1RVU1lTWkpTR0Q0USZlbmNyeXB0ZWRBZElkPUEwNDMwODQxMjdPRTdDVjU2WkdZMCZ3aWRnZXROYW1lPXNwX2F0ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=
3. https://www.amazon.com/Volbox-inline-audio-control-attenuator/dp/B071VL2GF8/ref=sr_1_5?keywords=attenuator+3.5mm&qid=1569248936&s=electronics&sr=1-5

I got the second one listed which is good but there are a ton to choose from some more tailored than others.