The shells used in divination are the natural shell of Cypraea moneta, historically gathered and traded in bulk along West African trade routes where they also circulated as actual currency for a period, a genuine dual history as both money and oracle rather than a purely symbolic object from the outset.
Preparation for divination
A shell intended for divination is filed or cut flat on its back, so that when cast it can land showing either its natural ridged, convex face or the flattened cut face, giving the system its basic binary reading unit, and a set of sixteen shells is commonly used in the Yoruba divination system called merindinlogun, a related but distinct method from the palm nut and chain based Ifa system.
Who casts them, and the ordinary use that carries no restriction
Casting shells for actual divination is restricted to trained, initiated diviners, often though not exclusively priestesses of specific orishas within particular lineages, a genuine skill and initiation based restriction rather than something any interested person may take up. A diviner's working set is a personal professional tool, cared for and given offerings much like other consecrated ritual objects, and is not lent or shared casually, a restriction that has nothing to do with cowrie shells used purely as jewelry or hair ornamentation, which carry no such limitation at all.