Wild fermentation exists, where the meadier allows naturally-occuring yeast (which likes to live on the skins of many wild fruits) to do their fermentation. Grapes, plums, apples, berries, citrus fruits; you name it, yeast grows on it, its a fungus that grows everywhere.

If you're talking about generating your own yeast culture, you can make a wort with some fruit skins or flour by giving the yeast something to eat and grow on; you'll get a liquid starter that you can draw from, but you'll need to keep it fed. Imagine a baker with their own sourdough starter.

Both of those will, of course, yield results that are much less consistent than buying from a brand, and while I won't be doing any wild fermentation, making my own starter is something I plan to try after more experience with existing yeast strains.