where can you buy NEW emergency brake cables?

Autozone also has them. Got my SVO cables there. The drum cables are :

BRAKE CABLE for a
1991 FORD MUSTANG

PIONEER

FITS EITHER SIDE CA-5237 $12.99

If you just have to replace the cable to the drums, no need to touch the handle spring. But if it is the center cable, then that is another story.

Good luck!