On the left side of the campaign map is 4 anchor symbols, these are trade nodes. These trade nodes give you resources such as Incense, cotton and, you guessed it, silk. The trade route second from top has silk, to acquire it send a trade ship to it and place it on top of the anchor symbol or alternatively find out which other clan has it and trade them for it. Bear in mind there are 2 other trade points, 1 in the middle with horse and one on the right side with iron.