This is mentioned most heavily in Dune Messiah and only once in Children of Dune, never in the original book, Dune.  In Dune Messiah you can find tarot references, specifically Dune Tarot references scattered throughout the entire book.  Here is an portion of Alia thinking about the Dune Tarot.

Red and green packages containing the new Dune Tarot were prominent among the vendors' wares, Alia saw. She wondered about the tarot. Who was feeding this device into the Arrakeen market? Why had the tarot sprung to prominence at this particular time and place? Was it to muddy Time? Spice addiction always conveyed some sensitivity to prediction. Fremen were notoriously fey. Was it an accident that so many of them dabbled in portents and omens here and now? She decided to seek an answer at the first opportunity.

Hope this clears it up for you.