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?

I think that wine yeasts are basically refined strains of the common bread yeast, saccharomyces cerevisiae. Actually a lot of traditional recipes for alcoholic beverages call for bread yeast, like beetroot sherry for example, because that's what regular people had access to in the olden days.

I experimented with wild yeasts.

If you can't get/don't want to use lab yeasts (dried in bag), the best bet is to start with about 1l of fruit juice or wort, put it in Mason jar or some container with larger opening and let it sit outside in garden or on a window over night.

Put it inside, let it ferment completely (it will take about 7days) in somewhat warm room (20 - 25°C). Taste it and smell it if it is good (no off smells and off flavors like acid, bitter or rotten). And use it as starter.

I messed with yeasts about 2 years ago with petri dishes, isolates, microscope and stuff but this basic recipe will do it. Bear in mind that it is about 50/50 if you get good or bad strains so make multiple batches and use only the most promising ones.

Yeasts are pretty much everywhere so you will most likely catch something, but you don't want any bacteria or mold.

I will try to find yt vids about this (someone recommended me sth on reddit) and post them there.

Not sure if the information is still current, as it was a while ago, but I was taught that the yeast for wine originally came from the grapes themselves, it settles on the fruit from the air as it ripens. They named it Saccharomyces ellipsoideus or something of the sort, related to the beer yeast S. cerevisae, but not the same. Nowadays winemakers use lab cultured yeasts for consistency though.

Point being, if you can get hold of some grapes that you know weren't exposed to nasty stuff (i.e. not grown near a highway), try not washing them and just smashing them, skin and all, and see what ferments. Put an airlock on to prevent acetic acid formation and try going from there. If using water to dilute or wash stuff, make sure it's chlorine free. One campden tablet to 20L should do it.

The fresh fruit should work. Are you asking if a bottle of wine would give you yeast? The answer is generally no. Most wine has stuff added to stop fermentation. It may also be filtered to remove yeasts, etc.