Any idea where I can find this cable?

[Plugers]

I think any old stranded 24-28AWG two-conductor jacketed cable should do it. Just make sure you like the way it looks. (not a joke)

Also, the thicker the jacket, the more annoying it'll be.

For the headphones, you'll want 3 conductor though. Right-left-ground (common)

Otherwise, yes you would only need 2 conductors per channel.

As for the jacket, if you're buying new, look for silicone if you want super flexible.