I picked up some sourdough starter when I was in San Francisco. With this
was made a huge batch of the stuff (one gallons worth). It was then put
into my food dehydrator (drier), an oven set at 100 degrees will do the same
(for 12 hours or more). Once it was all dried up, I pulverized it
and put it into a zip lock bag.
This was then used to restart more starter (and it did work). Or, added to
my bread for great flavor. Use one or two tablespoons put in with your
yeast. I have found best results if you allow both the yeast and starter to
sit in your liquid for 5 minutes or so.
Another source for a good sour dough starter is Knots Berry Farm in
Anaheim, CA.