The easiest way to do this is to take the broken spring to a local garage door dealer or home improvement center so they can help you find an exact match.
How to fix a garage door extension spring.
Spray each spring with garage door lubricant.
The stretching and contracting of the door spring on the garage door balance out the heavyweight of the garage door to help it slide up and down easily during opening and closing.
Wipe off any excess then repeat the process on the other side.
Clamp a locking pliers or a c clamp to the track just above one of the rollers.
Slide a grocery bag or piece of cardboard behind the spring to protect the garage door.
Purchase new garage door extension springs.
Shove a winding bar into a bottom hole of the winding cone of the good spring.
Loosen the unbroken spring.