Porcelain, unlike all other resources, doesn't appear on the land, but is instead made through the exceptional skills of the artisans in a Mercantile city-state. Thus, it is available only if you ally yourself with this city-state. You won't get this resource if you conquer or buy the city-state - the artisans will simply disperse and refuse to work for you!
Note that only one of the two special resources is available in each Mercantile city-state: either Porcelain or Jewelry. Note also that they are available alongside normal luxuries that the city-state will produce from the surrounding land.