You can buy this in some areas, especially the American South. Brands include Aunt Jemima, Martha White, White Lily, Tenda-Bake, Three Rivers, Yelton’s, etc.
Some brands such as Martha White may include some wheat flour (as of 2010.)
The White Lily Flour brand is owned by Smuckers since 2008. Milling facilities for the product have been moved out of the south in Knoxville, Tennesee, to the American midwest.  Some American southerners say they can tell the difference now.
Replacement suggestions include:
Per cup of self-rising cornmeal: 1 cup of cornmeal mixed with 2 teaspoons of baking powder, 1/2 teaspoon of salt.
1 cup cornmeal
1 cup flour
1 tbsp baking powder
1 tsp salt
1/4 cup of a fat (butter, margarine, shortening, bacon drippings or lard)
 Devwan, Shaila. Biscuit Bakers’ Treasured Mill Moves North. New York Times. 18 June 2008.