Murkmire Furniture: Murkmire furniture plans have a chance to drop from any qualifying containers in the Murkmire zone. Certain blue plans can also be pickpocketed in that zone.
Where to Farm: The unmarked dungeon Xul-Thuxis is hard to reach, but is the best (perhaps only) location to farm Murkmire plans without stealing. The Blackguard camp just inside the entrance is full of lootable containers, and better yet, the containers reset if you exit and re-enter the dungeon, allowing you to farm it endlessly on a single character.
Bonus: If you don’t have the time to travel to Xul-Thuxis for a farming session, there are a number of containers to steal from in both the Bright-Throat Village (check the Tree-Minder and Egg-Minders’ huts) and Lilmoth (check the market). Since blue plans can also be pickpocketed from NPCs in Murkmire, you can run a steady circuit through both settlements, picking and stealing along the way.