I haven't found them offline. I have bought them reasonable from some people you just have to email the order and they will give you their paypal addy and total. Matthews Poultry and Supplies. http://poultry2.tripod.com/id3.html
Wal-Mart and Staples have various sizes of shipping boxes, and the USPS have flat-rate priority mail boxes free, you just pay a flat rate to ship whatever you can fit in it. They are available online or at the PO.
I went through quite a few boxes shipping goodies to my son and daughter when they were in Afghanistan. My daughter has been back for a couple of months now, and my son is on the way home. YAY!!