why not create your own starter?
There are several methods including just flour and water and kept exposed to air. Enough yeast flying around that you might catch the right kind.
I suggest finding a cluster of organic grapes (kosher?) and soaking them in clean water for 15 minutes or more.
Remove the grapes and eat them. Mix equal parts flour into the water, cover and wait for the yeasts to eat all the sugars in the flour and grow.
The all you have to do is maintain your starter. oh...and bake great bread.
re: Food Tyrant
That's interesting. Just yesterday I started making the sourdough starter from the Moro cookbook. It calls for washing organic grapes, wrapping in muslin, and immersing in a mixture of 500g flour and 1 litre of water for a week to 10 days. I'd never heard of a recipe like this before, but I'll see how it goes. Not sure if it's kosher, though.