“What is buttermilk? Is this in liquid or powdered form? What can be used as a substitute and where can I get in?”

Buttermilk is the product that remains after milk has been churned to make butter.

Nowadays buttermilk is made by adding special cultures to skim milk, and is usually sold as cultured buttermilk.
It is available in leading supermarkets in powdered form.  Mix with water and use as required.
Fresh buttermilk has a very short shelf life and is seldom available in our supermarkets. You can use skim milk as a substitute, although the fat content in buttermilk is higher.

A recipe on Cinnamon Orange Scones that uses nonfat buttermilk.

Low Carb Diabetic Diet