Best Soil to make brown mud?

A peculiar request, but basically i’m in need to make some brown mud for a mud slip and slide. I need to buy some soil/earth/dirt off the shelf to do this, and not sure what the best option is? Of course it needs to be high in clay content, and low in sand so it can stick, without any rocks/chips or anything. I’ve been going round the houses for a while and really not sure what would be best?

If someone could direct me to some off the shelf soil that would be best for making this type of thing please let me know!!!