Where would I buy just the plastic cases that polyhedral dice sets come in?
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The Dice Shop are selling these: 25 x Standard Perspex Dice Cubes. I expect that some of the other specialist Dice Shops sell them too.
In addition to your normal game stores there is a brick & mortar store called the "Container Store" where I saw several differe nt sizes of them. (They weren't referred to as dice containers)
And ofcourse Amazon has them: Here is one I just found using my phone; I'm sure there are a lot more, its just harder to search Amazon on this phone.