My sister, who is like Mrs. Christmas Kringle, used to get her plastic boxes at CostCo. I think she now uses an online place called Plastic Container City. Both places sell them in huge quantities though. :/
Maybe a baker's supply store in your area? Or you may call up your local supermarket and see if you can simply purchase the number you need from their bakery.
A little bit Ginger. A little bit Mary Ann.