You don't need any bought yeast to start it. There is yeast everywhere (you're inhaling some right now). Just put a flour/sugar(can be granule or fruit)/water mixture out on your windowsill for a few days. And you'll have the same thing happen. The best part, wild yeast can be different every time you make it. Makes for different flavors. Just pick apples from the tree (don't wash them), quarter them, put them in a large container with water, cover with a tshirt (or something else permeable) and let them sit in the dark a few days. You'll start noticing bubbles, and finally a grey foam forming. The sugar in the apples gets converted to alcohol, and then the yeast turns it into vinegar. And you're left with the "mother" which is basically just a big gob of yeast and probiotics. Strain the solids out, put liquid and the mother into jars, and put it somewhere dark/cool. You can split some mother off and make a new batch (speeds it up a lot), or dehydrate some and crush it up. Good primer with photos: How to make yeast | Shopping in my basement
Very neat! The yeast doesn't turn to vinegar, though. A bacteria called Acteobacter causes the conversion of alcohol to acetic acid or vinegar. And yes that natural vinegar is probiotic!