Where can I purchase boxes for 2 and 3 tier cakes?
Have you tried Global Sugar Art? They have a wedding cake/special occasion box that worked well for me. One panel is perforated so it can be easily cut open to pull the cake out. I was a bit sticker shock at first, but the ease of use was priceless.
If it's just to transport it, I use the brown packing boxes they sell at Staples. A 16" cube is big enough for three-tiered cake to fit in totally stacked.