First of all, THERE IS NO SUN SoTNE IN PETALBURG!!! Don't go there. Wild Solrock hold them but the chances are unlikely. There is a sailor in the Mossdeep Space Center that will give you one if you talk to him. There may be more sun stones, but this is all I know.
In victory road
In order to get the Sun Stone you need to go inside the space center and talk to the old man. This stone will give you certain fire types. All this information is available on the following link. Please visit Pokemon to get a full walk through of the game.
In safari zone