Copper Wires can be stolen from the containers inside NPC Blacksmith shops. They are used by the tinkers to create a Distillery. One of the best ways to obtain these is to pick the Barrel and the Large Crates inside the Blacksmith shop in Serpent's Hold. Note that the contents of these containers should be stolen for them to respawn.
Copper wire was introduced as an item that can also be looted from scuttled Pirate Ship. They are uncommon (not on every pirate ship), and often come available as a stack of 20 wire.