I've just done this for something obscure where nothing is available off the shelf.
You take an old bit of outer, and run it in wide curves from the handlebar lever to
the brake anchor point, and measure. And with a bit of string, run from the handlebar lever,
and run in those same wide curves, to the brake lever anchor point, and measure.
That gives you the inner and outer lengths.
And figure out what fittings this will need at each end, to tell your maker.
I then went and bought a suitable cable which I knew was longer, and trimmed it down to suit.
https://i.ebayimg.com/images/g/VZIAAOSwRg5gGMH2/s-l1600.jpgBeing able to solder made this easier - its not difficult.
Getting the solder to actually stick is the trick - flux is important !
