Have I got a question for you! I am in the need of 8 earthquake treasure cards, the bazaar hardly has them, so to get to the question do you know a boss which drops earthquake treasure cards?
Ok, I'm going to assume this is for crafting something because if it wasn't, the easiest solution is player-made earthquake cards through an applied tough card. BUT, if this *is* for crafting, then, yup, I can see your problem because the usual earthquake+tough card treasure cards DO NOT work for crafting receipes.
The only cards that do work for crafting are the ones you craft through the school trees, drop from a boss, or that you get from the Eye of Bartleby or Marvelous Minions crowns card packs.
Here's the receipe for Earthquake cards through Ivan.
The Humongofrog component is available through the library, but that minotaur card presents a problem, yeah here's the minotaur card receipe as well.
And, you'll need to track down a Master Craftsman that's a Myth wizard to make those of course . . . any takers? Well, I'm going to guess that the best place to work out a trade or make a request would be over here on Wizard101 Central in their crafting forum. I'd be careful though, you really have to watch those percentages and damage numbers when making a trade to make sure someone didn't just apply a tough card to an earthquake spell and tried to pass it off as a true crafted treasure card.
cripes, I feel for you Tyler.
To answer your question directly . . . The only boss that Wikia lists as dropping Earthquake treasure cards is the Jade Oni. But, have I ever seen that happen? Um. No. Best of luck with that. You may also want to try the Dragonspyre myth bosses that cast the earthquake spell as well.