Dan covered this in his latest update:
But at the same time that we’ve been having record print volumes, we’ve been having record challenges in keeping Proofgrade materials in stock. Our operations team has been working long hours to keep our factory open, and can no longer travel to meet with suppliers. There’s a global acrylic shortage . Suppliers of wood, leather, and other products are experiencing their own challenges with staffing because of the crisis.
We apologize. We’re working to keep a variety of materials in stock, but we don’t know how soon stock levels will return to normal.