for one cup of soured milk (as a buttermilk substitute):
Warm milk in the microwave or on stove top
Add 1 Tablespoon fresh lemon juice or good (Heinz) white vinegar.
Let stand in a warm spot for about 10 to 20 minutes.
Cream of tartar-- 1 3/4 tsp to 1 cup milk--also works.
I have soured fat free half & half as well as 1%, but I understand that it differs from brand to brand how successful this will be.
There is also a powdered buttermilk that is sold:
http://www.sacofoods.com/culteredbuttermilkblend.html
and one from King Arthur:
http://www.kingarthurflour.com/shop...=epicuriousb&utm_medium=partner&utm_term=shop
Good Luck!
Jean