While we have collective nouns for animals (a herd of cows, a flock of birds), objects (a fleet of cars, a stack of books), and even materials (a pile of wood, a clump of clay), leather doesn't seem to have its own unique term.
The best way to describe a group of leather items would be to use a general term like "a collection of leather goods," "a pile of leather," or "a batch of leather."