What are the most yeasty fruits sold in supermarkets suitable for providing yeast for making bread?

Yeast on the surface of fruit and even flowers is a quite normal occurrence. For a start, I would recommend fruit that are commonly fermented for alcoholic drinks, so my first choice would be apples or grapes - also because they are quite easily obtained. Note that the yeast is on the skins, so using just peels or small fruit like berries or grapes is better.

However, any kind of “treatment” may interfere with the naturally yeasts on the skins, so I would recommend buying organic fruit and perhaps even skip washing them (ok, that’s probably debatable and personal choice). If you can get your hands on fruit straight from a garden or similar, that would be better than any fruit that’s been through a commercial packaging process and sat on shelves for a while. They may also have picked up mold spores that are harmless when you use the fruit as usual, but can increase the risk of failure for yeast harvesting. You also want fruit to be harvested as mature as possible, to get high sugar levels and subsequently high yeast levels - just to get things started faster.

During the recent Covid crisis and lockdown, some stores ran out of commercial yeast and while sourdough can be a substitute, yeast water had its renaissance, because it’s a faster method and can be made with fruit from storage. The probably most used candidates are raisins, but figs, dates and everything that gets dried with the skin can work. Note that raisins have a particularly high surface to volume ratio, so the yield is quite good. Apart from making sure you get the skins (which is theoretically enough, you don’t need the flesh for yeast), you also want untreated pure fruit, so neither sulfur-treatment (for „brighter“ fruit and preservation) nor those poached in sugar, like it’s often done for cranberries and other sour fruit. Not part of your question, but maybe worth an experiment: some flowers have a high yeast content, so soaking a few elderflowers in sugar water gives you the same results as the raisins, but it’s a seasonal thing - and I have read (but not tested) about linden and sunflowers as good nectar yeast sources.

A possible answer that is very practical for those who shop in British supermarkets is bilberries. These tend to be covered with a white powder which surely almost certainly contains a lot of yeast. I am in the process of testing this hypothesis.

Dried persimmon have a white bloomy coat or soon will after purchase.

This gal produces 'water yeast' for baking; details about her attempt with persimmon:

https://www.thefreshloaf.com/node/49639/park-tae-young-yeast-water-chronicles