The botanical source most often used for extraction purposes is Mimosa hostilis root bark.
Chacruna (Psychotria viridis) and Chaliponga (Diplopterys cabrerana) are the two most commonly used admixture plants for brewing traditional Ayahuasca. Mimosa hostilis can also be used in brewing.
You can extract from chali and cruna but they are a lot harder to work with and not to yield as well as Mimosa.
Check the forum suppliers section for sources of all these botanicals.