This is the closest statistic I could find:
>Everstone is the most common result, appearing approximately 50% of the time. The elemental stones (Fire, Leaf, Thunder, and Water) are the next most common, followed by all of the remaining stones. ~Bulbapedia
I would estimate the chances being very low. As far as my research went I found no actual statistic anywhere, so I think this is the best probability and best info available right now.
Alternatively, you can get a Moon Stone from PGL, Pickup or in Meteor Falls.