I've always wanted to get one of those hollow rubber balls about the size of a bowling ball and fill it with hypertufa and wait for it to cure in the shade. Someday I will do this just to see if I can make my own garden ball. If it works then I will experiment with different mixes and adding small pebbles and such, maybe with a tube in the center to use as a bubbler.

Couldn't you coat a bowling ball with something? Not sure what, but somebody has probably done it in the Garden Junk forum. Maybe ask over there.

There is a cement place just outside of Athens, down the road from the J&J flea market. We go every year when we make the "Athens Rounds" to Thyme after Thyme, and Goodness Grows etc. This is the least expensive place I know of. Excellent deals on all sorts of things, it's the only place I will purchase cast cement from. I get my pots for about $10 or so.
I'm with Eddie on making them. You could also just use a mortar mix to make 'em in balls. At some point I plan on making one for a fountain!

GGG

If you want polished granite nothing else will have the sheen and depth.
Several places on the web. And probally some of the exclusive garden centers.
A place that makes cemetery markers or counter tops might could help you.
I have several I purchased at Hill Street Pottery years ago. They are limestone though. And the algae gets into the crevices and ages well. But if want want that high polished looked granite would be beautiful.
Also I have several ceramic ones that look like marble. They were very plentiful a year or so ago.

Here is a link that might be useful: Spheres