The first spring is different than the rest,by the way it mounts. I stopped by a couple of upholstery shops looking for parts,did not find any.
But was told any spring in the seat could be used, just the mount in seat pan needed to be moved a little. Yes need both parts, spring and stiffener.
As you said the front foot has broken off the first two,also broke at third coil where spring would sit.
The spring wire is 0.175 to 0.180