Depending where you live, if it is temperate enough, there are wild yeast spores floating around in the air. Simply make a thick paste of flour, water, and a bit of sugar. This is kept in a glass jar, or crock. place cheesecloth over the opening and place on a window sill. The yeast spores will make their way into the paste (slurry) and begin ot multiply. After a few days, you will have sourdough starter. The reason sourdough is sour is that the wild yeast can withstand a more acidic environment than can commercial yeast. Acid s sour.
As yeast gobbles up sugars and starches, they produce CO2 and acid. The CO2 leavens the dough, while the acid flavors the bread. So, if you can't get your hands on packets, or containers of store-bought yeast, it is naturally present in the air. Live yeast spores have even been found in the vacuum of space, alive and well.
Once your sourdough has started, you have an inexhaustible source of yeast, as long as you feed it and take care of it.
Seeeeeeya; Chief Longwind of the North