Dry ice is readily available. Look in the yellow pages under "ice."
On a side note, bakers yeast, brewing yeast, grape yeast... will not survive the acidity of a sourdough starter. They will be long gone by the time the sourdough starter is cultured and viable. That is why the idea of using grapes to culture a sourdough colony, is laughable. Just one of the many myths attached to the entire sourdough voodoo thing. 
Yes but not practical. Not many are not interested in sourcing and storing dry ice. Not many are even interested in participating and all that is require so far is feeding a starter once a day.
I'm not understanding you here. We're not questioning if CY, IDY, or ADY will survive the acidity of a SD Starter. We are trying to see if an established yeast colony (cy, idy, ady, or starter) can be taken over by dormant or rogue yeast from the flour food source. If a SD culture does establish itself in my CY leaven kept at room temps, then we have our answer. That local dormant yeast presumably from the flour can take over an existing yeast colony.
Yes I know that a CY, IDY, or ADY leaven is not a starter but it still serves the same purpose for this test. I just thought it would be easier to use for detecting any new yeast culture growth since CY and commercial yeast are relatively flavorless.
I am also testing a mild starter at room temps along side the CY leaven.
That is why the idea of using grapes to culture a sourdough colony, is laughableWhy? I don't understand. Are you saying that it is not possible to culture wild yeast from grapes? If so, then this whole thread has been a waste of time. If we can't propagate a yeast culture from grapes, then how can we have any takeover from dormant yeast from flour or the skins of any fruit?
Did you read the entire thread? Please put more explanation into your posts so I don't have to wonder what you are talking about.
Thanks.