You can get the Leaf stone in Soul silver in these locations:
- National Park (Come 1st in Bug Catching Contest) -Catch a Scyther or Pinsir for a better chance of winning
- Route 34 (PokéGear Phonecall)
- Route 25 (Show Bill's Grandfather an Oddish)
- Viridian Forest
So the easiest way to get it would be the PokeGear Phonecall but you'd have to wait for that random call to happen and have the number of the person offering it. Since you're already at Victory road I assume you don't have the number so the bug catching contest is your best bet at this point.
Or you can wait until Viridian forest and get it there.
Oh I forgot to add, you can only get the Leaf stone post-National dex in the contest..sorry about that.
Hope I helped!