At the prison I was in (northern Indiana), dice were banned as "tools for gambling" -- yet playing cards were OK. Yea, it didn't make sense to any of the staff either.

We carved our own out of bars of "state soap" (the cheap Barker brand inmate soap that gets distributed in just about every institution). We carved out the little divots on each side for the numbers and used Kool-Aid to stain them. Some of the older guys who had been incarcerated for a long time were so good at making them[1] that they were damn near indistinguishable from the real thing until you felt them of course (they were made out of soap after all). They actually rolled fairly true as well.

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1: they could also carve out some very intricate chess pieces. I mean these were damn near works of art. You couldn't use state soap for those though, it's far too thin and brittle. You needed the good Ivory-brand stuff from the commissary.