Open cell foam
Open cell foam is soft, "squishy" foam used in some pillows. The cell walls within the foam are broken and air fills all of the spaces in the material. This makes the foam soft or weak, as if it were made of broken balloons or soft toy rubber balls.
Sources of open-cell foam include couch cusions, fabric stores, and POOF Footballs. Couch cushion foam, while often available for free on the side of the road, varies wildly in quality and is often unsuitable.
Computer packing foam is a specific type of high-density, charcoal-grey open-cell foam used in packaging that is useful for a wide variety of purposes, particularly including Constructing Arrows.