Joannes just gave me 15 which I am cutting down smaller to fit on my shelves. I Googled "corrugated plastic" and you can buy 48 48"x96" sheets for $33.00. I think we figured out we could get 6 to 8 sheets out of it depending on what size I wanted. They came in clear and colors. I like the idea of free however and since the fabric store uses them, they should be safe. Another thought is to cut down the priority boxes from USPS. They are a little lighter than the fabric boxes and may not take up as much room. Good luck. I know I want to get my fabric out of the plastic boxes and organized so I know what I have. Take care.
Wendy B