After failing to find replacement springs, I decided to simply clean up the existing ones. In theory, you are meant to replace the springs when you install new brake shoes. Ideally, I should have replaced them, but the old shoes didn’t have that much wear, so these springs won’t have been stretched too far.
This photo shows the magic of an ultrasonic cleaner, two springs before and after cleaning. Twenty minutes of cleaning, in a degreaser and the springs look like new!