Last nite after work I drove to Michaels (50 miles) for a 1/2 sheet cake box that I need for a last minute cake for this weekend. They were out of sheet cake boxes?? The largest they had was 12 x12. They didn't have any in the back. They sell a full aisle of cake decorating supplies and don't have boxes in stock??!! Man, I am really annoyed! I can't store 50 of them and there is no way to get some overnighted at this point. The local grocery that does cake won't sell them. 
I'm sorry-I feel your pain!! Luckily in my town, we have a Hobby Lobby right across the road basically from Michaels so if one doesn't have it, the other usually does. I don't have space for 50, but I have space for 10. Think about how many you might be doing and try to keep your own mini-stock. Cake boxes are thin. They can be slid under a bed if they need to be-just stick them in a big bag. If you are delivering the cake yourself, just carry it in this time-don't say anything about not having a box. Place it where they need it and nobody will say a word!
It is most likely the order system, which is auto replenish. I feel your frustration, as I teach at a Michael's and often I am missing stock in the Wilton aisle. I find we either have something or we don't. The way it works is is something is purchased, it should be automatically ordered. Problem is that by the time it gets there, we have run out.
beachcakes,
I feel your pain, and know your frustration 
My PITA is the local cake supply store:
Only had two 4 oz. bottles of creme bouquet (not the 8, or 16 oz. they said they had)
Asked if they could order me a GALLON, took my number and never called 
DIDN'T HAVE ANY strawberry filling (even though they said they ALWAYS have it in stock!!!! 
I was so angry and fed up that I ordered a gallon of creme royale (poor woman's version of creme bouqet) off the net and 50# of sweetex from Gordon Food Service, so I would NEVER have to go back there.
But that's just me.
Beach....I'm sorry this happened to you...what a pain in the neck!!! Did you try one of the bakeries by you? I'm sure someone will be nice enough to give you one.
Hmmm....maybe this is opportunity calling for you to open a cake-decorating supply place out there "on the beach". 
I'd visit ya!!
Where I'm located, there is a Michael's about 10 or 15 minutes away. Since they don't carry a large selection of of boxes or don't have the size you need, I end up going to Hobby Lobby in an emergency which is about a 30 minute dr. The cake supply store is about a 40 minute dr. I started ordering 10 inch boxes by the case. But other sizes I don't use that many and I wouldn't have anywhere to store a lot of sizes. When I do go to the cake supply I buy about 10 each of the other sizes I use.
Thanks guys! I feel better now.
BTW - I'm running out of room under my bed! 
DH is wondering why my hobby is taking over the house - I have cake supplies tucked all over the place!
Adven, I'd love to open a cake supply, but I don't think I'd do a booming business around here, 
how big is the sheet cake? what i do when i am out of 12*18 boxes i make 2 12*12 but i leave the 4th side open on both. put them in one another and open slide it out to the size i need and then staple it in place. yes you use two boxes per cake but you get whats needed.
Thanks Kjgjam22 - I'll have to try putting two boxes together!
vww104- If i had space under my sofa, believe me, there would be stuff under there! But I have a sectional that is on some sort of base :'(
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