You have to buy it!
it's rare to find a moonstone
you can not buy a moonstone in any Pokemon game but you can find them somewhere
you can find it any where
in space
A Moonstone can be found in Meteor Falls. According to Serebii, it is the only location to find a Moonstone in Pokemon Emerald.
Moonstone has been popular among Romans and Greeks and is even today a popular stone mined in various places in all the continents. The most famous of these moonstones is the blue colored Sri Lankan moonstone and the Indian rainbow moonstone.
you can find it in waterfall cave in kanto
In the underground they are very rare.
Not sure!
find its owner
first find a kecleon shop in a certain dungeon and than on the red rug it might have a moonstone get it pay for it (it is 1000$) and than you got your moonstone!
Mt. Mortar