Hero mode is the main source of hero soulstones. You can do each hero level 3 times per day and the cost is 12 energy per chance.
The Path of Heroes
This is a quest located at the bottom of the Normal quests list that gives [Yuffy] soulstones as a reward. The reason it is mentioned here is because we don't have a quest section on the Wiki, and it's not entirely obvious what this quest is about. Also, you won't start it accidentally because the first stage in Hero mode doesn't give any soulstones, so you have no reason to come back to it. To complete this quest (and its follow-ups) you have to play the stage mentioned on the quest in Hero mode twice, starting with "The Chief". Claim the quest before moving to the next stage.
This mode is not yet available as of 2016-02-15