best way to go?
1 Tablespoon vinegar or lemon juice into your milk of choice (rice, coconut, soy ... whatever). Should work. :)
Should have said into 1 cup of your milk of choice...
I second SeaJambon! It's a standard way to substitute "soured" milk products if you're dairy free.
I think they are looking for a non- dairy sub. Have you tried applesauce?
You can replace buttermilk with the same amount of coconut milk plus a Tbs. of lemon juice. You could probably try something similar with soy milk, but I think coconut milk would be way tastier.
Soured soy milk will probably work - it depends if the buttermilk flavor needs to prominent.
Nothing out there says you have to use buttermilk or milk to make cornbread in fact a recipe from Hominy Grill in Charleston uses water. Water works great and I use peanut oil too. You could also use palm oil which has become the fav for non hydrogenated shortening. I added several grinds of black pepper over the top before baking. I also find if you use corn flour instead of meal you get a much more tender cornbread.