Hey there,
It depends on luck if a Fire Stone appears in a minable rock, I guess you were just unlucky. As for evolving your Growlithe, I would wait till it reached the mid-40s level wise, so that it's still able to learn good moves.
You can find a fire Stone shortly after the 5th Gym inside Pyrous Mountain.
If you don't want to wait that long I could give you one, if you're interested.