What other sources are there for yeast without purchasing specific supplies of any kind?
I’ve done several lactose fermentation experiments and am currently playing with figurative fire by washing and running fruits through a food processor, letting them go active in a (burped) container and then adding them to other fruit juices. Currently I have a small apple for yeast that I added to pealed lemons and some lemon juice. I have no expectations for the results, and intend on buying nothing.
Natural wine maybe has some active yeast left in it. Many non natural wines will use metabisulfates to stabilize the wine. I imagine this isn’t beneficial to cropping yeast from it.
Other sources of yeast? It’s literally everywhere. Have you attempted spontaneous fermentation like Belgian’s do in lambic beers?
Yes. I’m currently on my 7th and 8th experiments with wild ferments.
I even did something of a fond with my first attempt using a mason jar of fermented blueberries in a 3% salt brine combined with the stock leftover from a 5 pound roast that spent 4 hours on the smoker first and 6 more in a dutch oven. That yielded around 1000ml of hardwood smoky flavored beef stock that originally started with chicken stock and a beer with garlic/onion/pepper etc. The blueberries yielded around 600ml of a savory juice that was like a slightly fruity soy sauce. I reduced that entire mix down to ~600ml and have my first real sauce. It is something like a smokey fruity Worcestershire (garum like fish sauce) sauce.
I’m also working with cherries, and a bunch of variations on a spicy lemon garlic/ginger to explore this area.