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.