Shops selling refrigerators and washers have the largest, thickest cardboard available. Since these boxes are a hassle for them to recycle, they often let customers take them for free.
Places like The Home Depot or Lowe’s sell large sheets (often labeled as "floor protectors" or flattened moving boxes) in the lumber or packing aisles.
UPS , FedEx , and U-Haul carry flat sheets designed for layering inside boxes or protecting mirrors and art during a move. 2. Craft and Office Supply Stores
The gold standard for industrial packaging. They offer dozens of sizes and "burst strengths" (durability levels), though shipping costs for large items can be high.
