Clay can be found along creek beds, in old pits and caves, along ridges where different stratification of soil have settled over time.
The nearabout of where she can be found vary from place to place and I imagine that if you spend long enough time in place there would be a potter or someone who has learnt the land to be able to take you there. I have been fortunate to live in places where such people were still around and had the opportunity of learning the difference between working with wild and commercial clay.
Not all wild clays are cleaned and ready for use. Some are but many require further washing and cleaning.
There is a man in Norther Italy who told me that before you are able to work with wild clay you need to let her sit to rest for a year once she has been washed. Such time is to let her rest and regain strength in her body. Quite an incredible request given the modern time we live in where everything must be ready for use right away at whatever cost and consequence.