Ancient Cloth

Ghouls patrol the Underground Desert once the Wall of Flesh is down, and they show up often enough that cloth piles up on its own while you are there for other reasons. For most of a playthrough that pile has nothing to spend itself on and just sits in a chest.

The Beheaded gear changed that. If you want that set, this material is what actually gates you, so it pays to sweep desert corridors on purpose rather than waiting for incidental drops. A lit horizontal tunnel through the desert layer with a few traps at the ends fills the gap far quicker than passive exploring.