Question

Where can Leaf Stones be found in Emerald?

Answer 1

I'm answering this question because the previous answer is wrong as it's about kanto not emerald.

You will need to find a Green Shard and trade it with the Treasure Hunter, his house is on a small island, west of Mossdeep. You will need Dive and you need to beat Liza and Tate (at least you're getting that Sharpedo ;) ). After that, surf west from Mossdeep. After you see the treasure hunter's house, go Dive on the dark water at the bottom left side if his house. Keep surfing, then you will see light, it will lead you to another area, where you need to dive again, but after diving, you will be back at an area where you find a Pokeball. It contains the Green Shard.

Hope I helped!

Source

Answer 2

There is a leaf stone in the safari zone.
http://www.serebii.net/pokearth/kanto/safarizone.shtml

Answer 3

In case you can't use dive yet, you can get a leaf stone on Route 119.
source