Where can I buy parts for this headphones?

I have these headphones forever, and I love them, I use it every day. The battery life still works the same way since the time I bought it. Unfortunately, after dropping it uncountable times, the left year control broke, detached, I want to replace it because the headphone is still fully functional, but I can’t find where to buy the spare parts. Anyone have a suggestion of where to find it? Thanks