The mission will give you countless ones, which are enough for you to use, or you can buy them in the trading field, or you can team up with others to fight, as long as they have the keys (you can also get them from the Abyss Island) . But if you are talking about the Abyss Badge AB or something, go for Rumble, Ice Slag, or buy it. D is easy to earn. I have earned more than 200 D without recharging.