I know Angelo Bros. used to sell the retractable cord ball assemblies, but since Westinghouse bought them out I can’t seem to find diddly on them.
I found a fixture from Pottery Barn that has a neat looking retracting cord setup with the pulleys and all. Maybe you could get the less expensive one and cannibalize it just for the cord set. The only thing is that it looks like it may be more of a pewter color than the copper you have, so you may need to do some detailing.
Light Fixture 
