I thought this too when I was looking for dice to generate private keys (actually, to generate electrum wallet seeds, but the principle is the same). But actually you are mistaken. The dice need not be platonic. I bought a bunch of 16-sided dice here: http://www.thediceshoponline.com/dice-search/16-sided. They need only possess sufficient rotational and reflectional symmetry, so that each face has an equal chance of landing up. It seems to me, the general idea behind the 16-sided models is adaptable to any even number of sides from 6 up (though for 6 itself you'd prefer a cube, and for 8 that gives you the platonic octahedron anyway).