Watch out for fake shamans there in Iquitos. It has become a very touristy place. Loads of guy's pretending to be shamans just to get money of you. Guy's that don't know shit what they're doing.

If you haven't bought your ticket yet I would seriously consider going to Ecuador instead. Less corrupted by tourism.

Check this: Ayahuasca Ecuador | Ceremonías y retiros de Ayahuasca en Ecuador

Well have a nice trip where ever you're going.