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The Oxford Companion to Beer definition of wild yeast
beerandbrewing.com

Although the microbiology of beer is complex, the wild yeasts most often associated with brewing are natural strains of Saccharomyces and Brettanomyces (Dekkera) and, to a much lesser extent, the yeasts Candida and Pichia, as well as other oxidative yeasts. Wild Saccharomyces may include both natural strains of the ale, wine, and bread yeast Saccharomyces cerevisiae or the lager yeast Saccharomyces pastorianus. Although S. cerevisiae is known to exist independently of human activity and can be found on a variety of substrates including the surfaces of fruits and plants, as well as in soil and tree sap, there is a tendency for it to thrive in environments rich in simple sugars, such as ripe fruit. Wild Saccharomyces yeast contamination is only encouraged in spontaneously fermented beers. Brettanomyces, which is the bane of winemakers, is also found on the surfaces of fruits and is generally avoided, except in spontaneous fermentation or when intentionally pitched as a pure culture. Candida, Pichia, and other oxidative yeasts in beer are always unintentional because these yeasts can contribute high levels of acetic acid when exposed to oxygen.

In Search of Wild Yeast
chelseagreen.com

Good Sources of Wild Yeasts
wild yeast starters• Grapes, plums, fruits that have a white bloom. From your garden or farmer’s market. Make sure they’re organic.
• Gingerroot. This must be organic—most of the ginger I find locally is imported from China and doesn’t work for fermentation. I suspect it’s been irradiated to kill potential microorganisms and insects.
• Fresh wild juniper berries. Note that not all juniper berries are edible.
• Elderberries
• Wild grapes
• Elderflowers
• Blueberries
• Blackberries
• Figs
• Prickly pear fruits
• Tree barks—birch (Betula spp.) and aspen (Populus spp.). Yellow birch bark (Betula alleghaniensis) works, too, but I think it should be collected in late spring or early summer.
• Unripe pinecones. My pinyon pinecones were loaded with yeast, and many people have reported excellent results using unripe pinecones from their local pines. You’ll need to experiment a bit.
• Pinyon pine or white pine branches. It’s likely that other species of pines are suitable as well, but I haven’t experimented with them yet.
• Raw honey 

A lot of unwashed organic fruits (apples, peaches, lemons, et cetera) are also excellent sources of yeast. Make sure they’re organic and clean (not grown in a polluted environment and sprayed with chemicals).

It was a pleasure to assist you with this task. Here is what I was able to find: 

DIYeast: Capturing Yeast
bootlegbiology.com
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Harvesting Wild Yeast
homebrewtalk.com
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Capturing Wild Yeast
hobbyfarms.com
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I Get Around – Wild Yeast Version
suigenerisbrewing.com