I'm sure this has been covered before, here's what I find the easiest way;
Surf east from Lilycove and keeps as far to the bottom as you can, once you come across a 4 square divespot, dive down, examine the light area and you'll obtain a green shard. Resurface, and continue east coming further north a bit, soon you'll get the shard masters house - exchange the shard for the leafstone.
There are others, but this is the easiest one for me to explain.