From Peter Dicken:
I'm no expert on finding clay, leaving it to the people who want an oven built to source it. Locally dug is always best, regardless of quality, as you can always adjust the amount of sand you mix in with it to compensate for impurities such as soil/sand in any clay you find. Look closely at becks/ghylls running down to the Eden. If the banks look yellow and firm it's more than likely to be clay as sand tends to drop when dry and form a beach.
The Little Salkeld Watermill clay came from their smallholding and is greenish in colour and contains quite a lot of soil. The clay for the oven at Acorn Bank, which came from below the wood leading from the house to the mill, is deep yellow, contains no soil and not too much stone/pebble.
To test what you find, mix substance with water; clay does not combine easily with it and becomes very sticky. Trial and error I'm afraid, is the best policy for us amateurs! Sand is easily obtained from builders' merchants / DIY stores and cheap; clay is very expensive once you start to try and buy the stuff, potters suppliers being about the only place you can buy it.