I have had a sourdough starter for many years, but it had been neglected lately, so I bought a new one from breadtopia.com.  It took about a week to rehydrate and build the starter to a useable quantity.  So far I have used it as a poolish replacement.  I have built it up to be 100% hydration (equal weights of flour and water) which will make it easier to use in recipes, since 4 oz. of starter consists of 2 oz. of water and 2 oz. of flour.