I believe you're talking about ayahuasca. It's a brew that's prepared by local shamans and used in "traditional" religious ceremonies, and makes people hallucinate and... you know, get in touch with their inner something. I know people who have used it, and it's not frowned upon. However, it has to be prepared by someone with experience, and since its effect lasts many hours those ceremonies usually take place at night.
Ayahuasca dealers don't exist, afaik. Marijuana and cocaine are cheap and quite easy to purchase, especially in Cusco (the city itself, not the village where Carla went missing).
I hope that was a little helpful