Lets see... From what I remember, your mother buys you one, after buying various other stuff first, with the money you send her. You can also find a moon stone in the cave with the waterfall when you first enter Kanto on route 27. Then on Mt Moon on Monday nights, you can rock smash the rock the clefariy do a crazy dance around to get one. I think you can do this every Monday night to get more.
And that is it.
The stone mom buys you is the earliest one you can get. Fastest thing to do is to give her all of your money and make sure that you have it set where money is sent to her after a battle. Go fight some trainers in an outside area and prepare for a call after each battle. Keep doing this until she buy the stone.
--- What's this about the Mocking Bird being the State Bird of Texas? Everyone should know it is the Mosquito.