Instructions
Speak with Bill's Grandfather in Fuchsia City. He will ask to see a certain Pokémon; if you show him the one he wants to see, he will reward you with a Leaf Stone.
Head to Johto's National Park on Tuesday, Thursday or Saturday and participate in the bug-catching competition. If you win the contest by catching the best bug-type Pokémon, you will earn a Leaf Stone.
Compete in events at the Pokéathlon Dome until you have amassed 2500 points. Visit the Athlete Shop on a Tuesday and you can purchase a Leaf Stone directly from the vendor.
Defeat Picnicker Gina on Route 34; she will give you a Leaf Stone as a reward for your victory. Gina battles with two Hoppips at level 9 and one level 12 Bulbasaur.
You can change the date on your Nintendo DS system menu to make day-specific items available, if you do not feel like waiting a week to purchase them.