Not sure if you are more interesed in the theoretical odds or the actual
making of sourdough with wild yeast, but according to this link, it is
actually not that hard to "catch" some wild yeast to make your own sour
dough starter.

http://www.io.com/~sjohn/sour.htm


John & / or Maryln wrote:
Hi All,
I decided I wanted to make sourdough pancakes so I found a
recipe for sourdough starter and I mixed up some flour and water and threw a
dab of bread yeast in and let it sit around uncovered for a while and sure
enough made some reasonable pancakes using my starter.Now I read a little
more and find that I didn't really catch a sourdough yeast because as I
understand it a sourdough yeast is wild and when I added the domestic bread
yeast I actually insured that I wouldn't have the wild one which might or
might not have been good tasting so I suppose that if I were to start over
and not add the bakers yeast but rather try to attract a wild yeast that
tastes good, well if I try it and don't like it I just throw it on the
compost pile and keep trying. Well, my question is what are my odds? How
many times do you suppose I'll have to try before I get a good one? Also if
I leave my quasi sour dough starter [ the one where I added the bread yeast]
uncovered will it become a mixture of bread yeast and wild yeast?

Thanks in advance,

John