I was told rubbermaid containers, turned upside down, make great cake boxes. Use the lid as the bottom. I put one of those non-skid pad things under the cake board (the kind you find in the kitchen section-just cut it to size) to keep it centered in the carrier. Then pop the "lid" on it. Works really well when you are going with the cake so you can bring back the carrier.
I use the regular bakery boxes and just tape the lid open as much as I need. Cover the opening with plastic wrap and tape it well to keep air, etc, out!
Here are a few links you can find tiered cake boxes...
http://www.sugarcraft.com/catalog/paper/boxes.htm#sq
http://www.winbeckler.com/cake_boxes.asp#Cake%20Boxes
www.Instawares.com
http://www.usbox.com/Candy-Food/index.html
Hope you find what you are needing!
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