There are 73 creatures that drop Humongofrog treasure cards. I have info on, um, well, 39 of them. I can tell you that you are mainly going to find it dropped in Savarstaad Pass, Grizzleheim; Krokotopia; Spiral Cup Gauntlet (Tier 5 - some of the critters on Tier 4 drop it too) and assorted areas in Celestia.
Click here for my current list of who drops Humongofrog Treasure Cards, depending on when you see this, there could be more than 39. Several of the critters are on my to-do list, but Duncan hasn't gotten around to them yet.
I would probably start with Risen Djeserit in Djeserit Family Tomb, Krokotopia; Krokopatra in Well of Spirits, Krokotopia; Bloodwing in The Floating Land, Celestia; or Shambling Ahnic in Ahnic Family Tomb, Krokotopia. These four bosses also drop pets and some other interesting items. Grimsnes BurntBlack is also an option right now. You can find him in the Wizard City Telegraph Box.