It's probably 7mm cable, but you could measure the diameter. Wire core cable is best, rather than resistor core. Use some dielectric grease smeared in the opening and on the outside of the cable when you screw it in the coils and caps to make turning easier and to seal out water.
Hotrod shops have fancy coloured HT cable if that's your fancy. Most auto parts stores here in Canada do not have copper core or wire core cable and I can only find it at hotrod shops like Performance Improvements.
