Took a baking class from world class chefs who were known for their breads. They recommended setting a sponge (precurser to bread dough) outside or in the kitchen for a few days to gather and culture yeast from the air.

If you bake alot, there will be yeast spores floating around to easily start a yeast culture from your kitchen countertop. Outside there are yeast spores floating around as well. My grandmother told me about women in her town that would share yeast cultures. (the kids who ran the errands would taste it because it was basically sugar, flour, and water - sometimes so much was tasted that it would be gone by the time they arrived at the door)

I wouldn't worry too much about yeast, but I have sealed packages in my freezer for convenience.