Look for the raw material first
A request for leather can mean two different things: finding the raw hide or processing it into the leather required by a recipe. Decorative objects with “leather” in their name answer neither question. Match the ingredient record before planning what to gather.
The release index has separate records for 
Light Hide, 
Medium Hide and 
Heavy Hide. Their item pages list recorded creature-drop sources. Those source lists identify where the data connects a material to a creature; they do not by themselves establish a guaranteed drop on every kill.
Process the matching hide
Light, Medium and Heavy Leather each have their own processing recipe. Open the recipe for the leather your finished item actually requires. Similar names do not make the three materials interchangeable.
Continue to the finished item
Once you have the required leather, follow its uses or return to your target recipe. The Iron Sword guide is one example of a chain that includes processed leather as well as metal.
This article describes the relationship between records. The live recipe cards show current indexed inputs and processing requirements; the source section records the revision used for the explanation.