Moon stone FireRed

There are only a defined number of these in the game (I think it is like 4), and they can be found in mt moon and other various locations. I also think that there's a small possibility to have a clefairy holding a moon stone when captured (like 7 % or less), but since clefairys are hard to find, I wouldn't waste my time on trying to catpure like 20 clefairys.