It looks to me as if those are foam core boards -- very light and widely available in places like Walmart, Target, etc. They would likely be in the school products area where posterboard and paper are sold. The boards are about a quarter-inch thick with white paper on both sides. They also come in black.
Just be aware that the paper on these would not be acid-free (archival quality). If using them to store fabric, you might want to coat them with polyurethane.