I make beer
and mead. Mostly we buy yeast from known suppliers.
However, in a pinch, you can make a dough, and leave it set for a few hours out in the air - give it a good mix and wait again. Eventually it will rise. Yeasts' cells are wind
borne and can be found EVERYWHERE....
the BAD thing to doing this is bacteria can take hold too, but USUALLY yeast will produce enough alcohol content quickly enough to prevent bacterial infection.
Anyway - I said in a pinch, I wouldn't make a practice of this. haha.
Also, my wife would recommend making a batch of bread and keeping some of it as a starter batch. It will contain enough yeast cells to reproduce in bread dough in a couple of hours or so to give you bread that rises.
(Of course, I'm neither a chemist, nor a DNA analyst nor do I play them on TV, but my Stout is AWESOME!)