Hugh
What about a big glass Christmas ornament ball? Paint it the color you want.
Next thought is about the same, but using a 9" play ball.......rubber or plastic?
What about getting a 9" rubber ball....put a little paper mache over it.....smooth it out....then put a coat of fiberglass over the top....paint the color you want.
How about hollowing out a bowling ball. Make a donut jig to hold it while hollowing. That would lighten it a lot.
You could always use gilding on the outside of some ball to make it look like metal.
Hugh