I need one to evolve my kirlia.
In pokemon Diamon and pearl you can get it in Mt Coronet, using surf, (in route 208) also in route 225, using Rock Climb.
In Platinium you get it in the same parts as diamond and pearl but also in the south part of route 212 (you have to search in part where is mud).
you can also get in the The Underground.
and a pokemon with the hablility Pickup (the pokemon has to be at least at level 51 to get a dawn stone)
Down from eterna city, down the bicycle path and right into mount coronet.
go south and you should see a small lake. surf the lake and you get back on some ground. go right and you should see a pokeball. thats the dawn stone.