I am repairing a very old platform rocker and find that the zig-zag springs are broken. How can I replace them and where can I find them? I can find coil springs on a steel bar--could they replace the zig-zag springs?
If you can locate coil springs on a steel bar that is the better way to go. They are tied like coil springs with the exception that they are not on jute webbing. Springs on steel bars were in use long before no-sag (zig-zag) springs were invented and they were often used on platform rockers because there is no place for jute webbing on platform rockers - so most likely you are just going back to what the chair once had, I believe that it is the coil springs on steel bars and not the no-sag springs that are original to your chair.
Should you decide to use no-sag springs anyway then you can purchase no-sag springs by typing "upholstery supplies" into a search engine and several sights will come up. Use the heavy gauge which is for seats, light gauge is for backs - there are only two gauges. The clips to hold no-sag springs tend to crack and split the hard wood on platform rockers.
Installing no-sag springs can be extremely dangerous and cause very serious injury be extremely careful with them.