I live in Central America
and haven't see self-rising cornmeal or buttermilk in years. As a Southern Belle I missed my cornbread and buttermilk biscuits. Here is what I've learned:
You can make a buttermilk substitute by adding 1 tablespoon of white vinegar per cup of whole milk. Let it sit for 10 minutes. Excellent for baked goods like biscuits & cornbread, add an egg for a 'wash' and dip your chicken fired steak, fried chicken, Okra, etc. in it before dredging in flour.
And as far as 'Martha White self-rising cornmeal' it's easy to substitute that as well. Use a ratio of about 1 part all purpose (white) flour, 1 to 2 parts
corn flour (masa harina) to 1 part Polenta. Vary the Polenta/masa harina ratio to your liking for coarseness or 'grain'. Add 1/2 tsp of baking powder and 1/8 tsp baking soda, 1/8 tsp salt
and 1 tsp sugar per cup of flour. Store in airtight container in cool/dry place. then all you need is the wet ingredients (milk, egg, oil) to make cornbread
For example, the flour ratio for my 'Southern Style' cornbread recipe for an 8" cast iron skillet is
1/2 cup all purpose flour
1/2 cup Polenta
1 cup masa harina (corn flour)
1 tbsp baking powder
1/4 tsp baking soda
1 tbsp sugar
1/2 tsp kosher salt
3/4 cup 'buttermilk'
2 large eggs
1/4 cup corn oil
to 425 deg. Place cast iron skillet *with oil* in oven
to preheat. Put slight tsp white vinegar in 3/4 cup of milk. Let stand while mixing the rest of the ingredients.
Use two cups of the 'Martha White substitute' or follow amounts above. Add eggs to 'buttermilk' and mix well. Add egg/milk mixture to flour/masa/polenta mix. Stir until just combined (do not overwork). Once oven has reached 425 deg remove skillet (careful!) and pour hot oil
into batter (careful!). Stir well and pour batter into hot skillet (careful!). Preheating skillet creates a crispy brown crusty bottom & sides typical of good ol Southern cornbread.