If you would like to make your own homemade Sourdough I have the recipe for you. This will give you a easy, quick way to whip up some sourdough for a sandwhich. Here is the wonderful recipe!

The night before baking, mix in a very large bowl a batter made of:
       2 cups sourdough starter
       4 cups lukewarm water
       5 cups flour
Mix well, although there may still be small lumps.  Cover lightly
and leave overnight at room temperature.  The next morning, stir
down the batter and return 2 cups to your permanent sourdough
container.  Add:
              3 cups lukewarm water
       1/4 cup sugar
       1 tablespoon salt
       1 cup powdered milk
       1/4 cup margarine or butter, melted and cooled (1/2 stick)
                 or vegetable oil
        flour (white, whole wheat, or a combination
               thereof; up to 10% other flours may be used)
Stir in about 5 cups of flour and beat well.  Add about 5 or 6 more
cups gradually, until too stiff to stir, then turn out and knead
well, adding flour as necessary until the dough is smooth and
stands about 1/3 as high as it is wide when resting, or more.
Place in a greased bowl, let rise until double.  Punch down, let
rest 15 minutes.  Shape into 5 loaves, place in greased bread pans
(9 x 5 x 3).  Brush tops with 1 tablespoon melted margarine or butter.
Let rise until tops are almost even with top edge of pan.  Bake 45
minutes at 375.  Turn out immediately onto racks.  For a soft crust,
rub with hard butter or margarine while still hot.  Freeze in plastic
bags when cool.
For one or two loaves, you may use the following amounts, per loaf:
 1 cup fresh starter
 1/2 cup warm water
 1 tablespoon sugar
 1/2 teaspoon salt
 2 tablespoons dry milk powder
 1 tablespoon vegetable oil or melted shortening
 flour to make a stiff dough (2 - 3 cups)