Store bought yeast can be replaced with simple and easy to make home made yeast (sourdough).
Making the sourdough
Sourdough reciepes can vary a lot. Simplest sourdough reciepe is made from flour and water.
In a small recipient (eg. jar), put one part water, and one part wheat flour. Whole or bread flour work best, but any wheat flour will work. Mix the water and flour untill homogene.
Even if not sure about the right proportions of water or flour, in the end, the mix has to be a little more consistent (like regular dough).
Stir every few hours.
In about two days, the dough will start foaming.
The sourdough is ready to use.
Things to know when baking with sourdough.
The starter can get a sour taste and smell, but it still can be used.
It will take more time to grow than the regular yeast (usually up to few hours).
Keep the composition, in a warm place untill ready to bake.
When composition has grown double size, it is ready to bake.
Other variations from this basic reciepe, represent replacing the water with juice
from fermenting various fruits, or vines, beer or adding sugar.
This will short the time needed to form the starter.