I have been through the wheel search. They turn up. Also helpful is a vendor who can weld, true, and refinish as needed.
Here's a discovery that may be of use: I have purchased silver (ball finish, no paint on the wheel), silver paint, and gray paint wheels. When you take the paint off of the painted ones, you have a ball finish wheel underneath. At least in the six or so that I have caused to have their paint removed. This greatly increases the supply of ball finished silver wheels as you can buy any color in the right size and get the ball finish result.