Piano wire, McMaster-Carr has it. Your shutter, by the way, is behaving normally. Go take some pictures, it's not worth the bother of making new springs.
Originally Posted by Tom1956
A properly made spring doesn't go bad anyway. I have and use several wtches 100+ years of age, were springs to weaken with age the rate of a watch (or anything controlled by a balance and balance spring) would become slower and slower. The watch in my pocket today is 101 years old, it has not changed it's rate in the 25~ years I've been using it.