you may try the link below which explains how to prepare a homemade oatmeal shampoo all by yourself.
http://www.doggiebuddy.com/homemade-oatmeal-dog-shampoo/.
You may also try these online shops which sell oatmeal shampoo for dogs
http://www.petsmart.com/product/index.jsp?productId=3687705
http://www.petco.com/product/103697/AvoDerm-Natural-Oatmeal-Shampoo-for-Dogs-And-Cats.aspx