<<Disclaimer: Verify this information before applying it to your situation.>>

I had a lot of very helpful responses about dry buttermiilk replacement.
Here they are:
Go to:   http://rdz.acor.org/lists/celiac/milkalt.html
You will have a list of dairy replacement

Buttermilk is interchangeable with sour milk or yogurt and a little bit of
vinegar.  (not dairyfree though).

Dry soymilk powder and 1 tsp vinegar

Sovex better than milk tofu powder

Mashed potato flakes.  Tried it and didnt see difference.  Have to wix well
each time beccause very llight.

Hope this helps everybody with dairy intolerance,
Mireille.