How do you find leafstone in crystal?

Answer 1

If you show Bill's Grandfather, in Route 25, an Oddish, he will give you a leaf stone. In Crystal only, you can get one from Picnicker Gina. But you'll have to wait for her to call you. She also gives other stones.

Answer 2

Buy one at the Celadon City Dept. Store during the second half of the game (Kanto region). It costs 2100 Pokédollars

Or

Defeat Picnicker Gina on Route 34 and accept her phone number. She will eventually call you about an item she found. If you speak to her after receiving the call, she will give you a Leaf Stone.