When yeast can't be found in stores:

Watson Caran told HuffPost, “To create a wild yeast starter from nature, my suggestion is simple: 1/2 cup water and 1/2 cup flour, put in a wide mouth quart-sized jar.” Watson said to be sure to use filtered or spring water since chlorinated water will kill the yeast. Next, she said, “mix well and cover with a cheesecloth or plastic wrap. About 24 hours later, feed the starter with another cup of flour and enough water to get the same consistency as before.” She said to look for bubbles, which will indicate the process is working. The starter will be ready to use as a yeast substitute when it floats on top of the water in the jar.