If you don't have your own sourdough starter, there are a number of ways to get some.
You can find a friend who makes sourdough or keeps starter. If you start asking around, you might be surprised by how many people have sourdough starter sitting in the back of the fridge. You can buy a little kit of dry ingredients and mix up your own starter. Doing it this way will add some time because you'll need to let the starter get established before you use it.
Or, if you're feeling brave, you can try making your own starter from scratch. Here are two different methods that I've blogged about:
Here are two Instructables for making sourdough starter:
You can also buy sourdough starter from someplace like King Arthur Bread flour
I've blogged about sourdough starters and suggest that you read a little bit about sourdough starters. The more you understand about sourdough starter, the easier it will be to use it. These posts should help you find your way around the sourdough neighborhood:
The Care and Feeding of Sourdough
What NOT to do with Sourdough