Maybe antique-hardware.com has one that will work? Not sure if they have this style, but they have replacement parts for platform rockers.
I also saw something a long time ago about a similar problem (but it had no photo) and they had one made by an auto repair shop. You may have to find a place that will copy yours and make a new one.
The other option would be to find a chair that is beyond salvage, but has a good spring, and use that one.