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.
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. where can you buy cardboard sheets
Ask the staff for "slip sheets"—the thin cardboard used between layers of stacked produce or cans. Places like The Home Depot or Lowe’s sell