Lincoln provided the link to mathematician James Grime's writeup in his notebook. For anyone wanting the physical dice, they are available at https://mathsgear.co.uk/collections/dice/products/non-transitive-grime-dice . I first saw the Grime dice in a Royal Institute presentation What Happens When Maths Goes Wrong? - with Matt Parker. I have cued the video to the start of Matt's discussion on the non-transitive dice, but I highly recommend viewing and sharing the entire presentation. AFAICT, non-transitive systems are almost never discussed in high school classes; it's rather excellent to have an example to get one's hands around -- and visualize in Mathematica. Thanks, Lincoln!