Each tier of locations has a new set of resources- Leather, Wood, and Dye are all in the Chapter 2-3 tier (Warehouse and Docks); Silk, Ore, and Marble are found in the Chapter 4-5 tier (Magic Gardens and Cathedral); and Diamond is found in the King's hold, aka the final tier. As for silk specifically, it is found in the Magic Gardens, though I can't recall which section it is found in at this time (want to say lower left-hand corner, with ore being the bottom middle next to it, and marble being in the ruins at the Cathedral)