Brake cables are standardized. Basically any brake cable you get will fit. There will be 2 different ends on the replacement cable to match 2 standards for the ends. You cut off the one that you do not need and use the rest. Best, find a video that shows how to replace the cable. You cut off the excess length of the cable after you put it in, leaving a bit of excess for any adjustments. There is a cable tension adjustment barrel by the brake lever that you screw in before putting the cable in. Cutting of the cable needs to be clean to prevent fraying.
There are different considerations concerning the quality of the replacement cable you need to get. Since you will surely mistreat the cable on your first try, you might want to get a cheap cable. On the other hand, the rear cable is long and takes a tangle route on a folder which calls for a quality cable. People who work on their bikes usually keep a stock of brake and shift cables as it is necessary to replace them every once in a while.