I would try calling Mark Butterworth at http://butterworthclocks.com and also Steve at https://timesavers.com
I think you can purchase the center wheel from them but not sure if it comes with the compression spring. They might sell the spring separately so might be worth the time to give them a call. Other than that pulling it off of an old movement would be the only thing I can think of.
Hope this helps. Good luck Jason, Dan