What are the best plants to cultivate yeast from?
Yeast colonize the surface of many plants — for example they are a typical part of the normal surface flora (microbiome) of fruits such as grapes1 and pears2.

As you can see from the second reference, yeast can compete with other microorganisms to delay rotting of the fruit.

You might also find the "Uses" section of the wikipedia article on yeast and this article on winemaking to be of interest.

References:
1: Zahavi, T., Droby, S., Cohen, L., Weiss, B., &amp; Ben-Arie, R. (2002). Characterization of the yeast flora on the surface of grape berries in Israel. VITIS-GEILWEILERHOF-, 41(4), 203-208.
2: Benbow, J. M., &amp; Sugar, D. (1999). Fruit surface colonization and biological control of postharvest diseases of pear by preharvest yeast applications. Plant Disease, 83(9), 839-844.