I have a cake supply store that is close to me that I get mine from. 
Also found that in an emergency *this past weekend* that you can find gift boxes at Walmart that are cake sized... I found a 19x14x4 box to deliver a cake in. Saves you in a pinch! 
Not as pretty as those from the cake store, but they'll do.
I buy mine at a cake supply store too. I used to just buy them when I needed them but I've found that it's much easier to have some of the typical sizes on hand. So usually when I got I'll buy 5-10 at a time. Usually when I make a cake for someone I have most things on hand, rarely need something just at the cake supply store, so why go just for a box?!
If it is a fullsheet or something that large, I go to a local bakery to get the box.. but for the smaller boxes (1/2 sheet and smaller) I went to the bakery at walmart and they sold me the box and the board for a dollar.. not too bad, but then again I dont do that many right now.. nor do I have the space... poor dh is losing floor space as we speak..
I buy them at one of the local cake supply stores. . .
I bought 100, 10x10x6 boxes for $25.00 I think, maybe it was less.
If you have a cake supply store close by, then buy as you need them.
Or if you know you're going to be using A LOT of a particular size, then
spend the $ & buy the pack, it's more economical in the long run. . .
I try to have a variety of boxes and cake boards handy so I don't have to go to the cake supply store too often 
I know, can you believe I try to stay away???? Shocking isn't it? Well, the reason is, I'd buy the whole freaking store every time I go!!!!!!!!!! 
I need to win the lottery. . . 
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