I going to be getting platinum soon and I have been planning a team. I wanted to use Gallade, but I'm not sure if I can get a dawn stone early enough for him to be usable.
The earliest place would be in Route 212. Unfortunately, the Dawn Stone it hidden.
>In the swamp, near Pastoria City (hidden) -Bulbapedia
Other non-hidden places would be Route 225 and Mt. Coronet.