How to make Yeast

I was talking about yeast recently and a question popped in my head:  Where does yeast come from and can I make it?

So I did some research and found out that yes, I can make it with very little effort on my part.  The process is very simple and requires very little ingredients.  Here are some sites that I found (the processes are different for each so look at all of the links):

Natural yeast using raisins, water, and a glass jar.  Or here's one using strawberries (and I assume just water).

Airborne yeast using flour, sugar, warm water, bowl and cheesecloth.

Yeast substitute using baking soda and lemon juice.  Good to know but it looks like it's definately not first choice.