You need to get to sentinel island (available after completing Maxos temple) and then do some stuff and take over the battle tower to get the dragon stone.
If you have gotten that far, from the throne room the first time you take the doors out to the trainer's platform you take the dragon stone in a cutscene, and after you repel the attack on the tower, it will be in your inventory (you can use it from there, the pause item/skill menu, or assign it to a hotkey).
If you are not already past there, your first encounter on sentinel island you can mindread to go up a level, so the higher level you are (I made it to level 18 in the fjords) the more of an experience bonus that will be. You don't need to get to a high level, but it would be better if you got to a new level before entering, to get a full level bonus.
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