I've seen a lot of questions about where to find specific cards and was wondering if there was a good place to explain the mechanics of it to everyone. As per Mismagius Star's suggestion, I figured I may as well make a blog post about it. It seems to me that there are no pre-defined card drop locations for any Rare cards, only uncommon and common cards that are particularly useful against a certain enemy faction. Furthermore, it seems there are several different places it is possible to get rare cards: daily chests, quest chests, defending the stronghold, finishing top 5 in your league, the arenas, boss levels in the towers, and card packs, all of which are random. If any one wants to confirm this, or knows any other information on how to get certain types of cards, feel free to post a response!
PheonixPenguin