Dear Josie:
Check out the Fleischmann's website -- Fleischmann's makes a powdered
sourdough starter, and I believe the website gives information on where to buy
it. Otherwise, it's really not that hard to make your own. The recipe I used
was:
2 cups bread flour
2 cups skim milk
2 tsp. yeast
Mix the above in a ceramic, plastic, or glass bowl and let it sit at room
temperature, covered, for 7 days. Place in the refrigerator thereafter. Use
at least once every two weeks, and "feed" your starter with the same amount of
water/milk and flour as the starter you use; i.e., if you use one cup of
starter, feed the starter with one cup milk/water and one cup flour. The
author of the above recommends alternating water with milk when you feed your
starter. After feeding, let the starter sit at room temperature for 24 hours,
then place again in the refrigerator. It may take a few weeks for the starter
to be sour enough. If you want it to be more sour, add milk more often (or
yogurt); less sour, add water more often.
Hope this helps!
Corina