You don't need a fancy bag to make a story sack! Even a simple paper bag will do. Just decorate the outside of the bag, fill it with the book and other goodies and viola! I have gotten some feedback about how well a laminated folder or large envelope also works.
If you want to get a bit fancier, go to your local craft store and by and inexpensive canvas sack. These can be decorated using fabric paints and other craft products.
If you are real ambitious, you can make your own custom-made sacks. Here are some easy tote bag patterns!
What the bag looks like doesn't matter too much, although you do want it to look inviting to the child. It is what's inside the bag that counts!
The next step is to Pick a Story...