This sphere has a 24mm radius.

I couldn't find any spheres on here with a high enough poly count for lead casting, so I made one.

Edit: I added another file, this one with the sphere cut in half, as printing the ball on my solidoodle2 did not come out nearly as clean as I wanted. Even with supports the bottom half of the spheres were a mess, so instead I printed the hemispheres (flat side down of course) and they came out perfectly.