Like many resources, Insulated Wooden can drop from enemy NPCs. However, it is pretty uncommon, and even when it does drop, it only does so in tiny quantities. Usually, this is the case for sought-after resources like Insulated Wood, Bricks, etc. These drops are more designed as a small bonus if you're in need, or just extra junk to sift through in the inventory of a dead NPC. Relying on these drops isn't a sustainable farming method at all, you are much better off crafting Insulated Wood the intended way, as I've detailed above.