big box store have all size boxes but you have to get them before they are cut up...make friends with a stock person at a few stores and you will get many sizes...for free
craigs list is also a great source for bigger boxes for free.
I use many recycled boxes and turn them inside out so the outside is free of writing.
it is girl scout cookie time so they have plenty of the case boxes that are a great size and sturdy too.
post office also has video size box that is smaller but will still be about 4.20 to ship.
I am going to check out the supply store! great idea. thanks