Breastplate of Kings can be learnt at the Armoursmith in Ironforge and Shattrath, it requires 350 smithing to learn/craft. Bulwark and Ancient Bulwark require 375smithing and can be learnt at the same places. You need the previous one to make the next upgrade.
Id reccomend you to not learn the recipe for the Ancient Bulwark or maybe even the Bulwark if you don't have mats, they cost 22g - 45g to learn, might be a prob if your poor like me =P. Read up the mats on Alak or Thott ;)