*edit!*
The following shpeel is here since you seem to be creating this mod yourself, and seem unaware of how to culture yeast. The information is not terribly easy to find, and very few if any people still produce and maintain a "bread sponge" for making bread everyday in this modern age. I have put it here for your convenience. Use, or ignore at your leasure.
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The traditonal method of culturing yeast is to mix starch, flour, and water together into a watery slurry, then place it in a warm and dark place.
For instance, a simple bread starter can be created from 8oz of potato water (water that has had several freshly sliced potatoes washed in it) mixed with 1 tablespoon of flour.
When it starts to foam, it is stirred and divided, and used to make bread. The initial starter takes about 2 to 3 days. If at any time it smells rancid instead of fermented, discard and try again.
For alcohol, you allow it to ferment. (You want a different yeast for alcohol than you want for bread. The fermentation creates alcohol that kills the codeveloped bacteria and bread yeasts. The brewer's yeasts will survive up until 2% alcohol.) When it ferments, you filter, then use to innoculate decontaminated starter pots. The heat sterilized starter is innoculated with the alcoholic fermentations of the initial culture, and the disproportionate brewers yeast will take firm hold and outcompete the other yeasts.
Other sources of yeasts include berry skins, such as grapes. The white blush on grapes is actually live yeast living on exuded sugars from the grape itself. Yeast also grows on several species of berry, including raspberries, and gooseberries. (This means that wines should not require a yeast culture. The fruit comes with one already, unless you are obscessive about washing it before juicing.)
You could harvest a kind of brewers yeast culture from a previously produced alcoholic beverage, such as a first stage bottle of wine. The yeast would have to be gently cultured to avoid having a nasty flavored brew though. (Wine yeast is different from cereal yeasts.)
Regardless, sourcing yeast from mouldy rocks is incorrect.
(Is kind of a bizzare freak for preserving old folk knowledge and lost arts.)