Also here's a tip. It's easier to get them at lower standard duelist levels than higher ones.
This is because of how the drops work - the pool of potential reward cards gets larger as the SDs level up, since more cards get added. This means a lower probability of getting the card you want when there are so many potential cards to pull from.
That doesn't mean it's impossible to get one from a level 51 duelist. It's just rarer than getting one from a level 10 duelist.