There is no such thing as a Native American shaman. That's what we call "plastic medicine people". You cannot become a North American Indian medicine person without being within a tribe. To call yourself one is like trying to call yourself a Catholic Bishop without being associated with the Vatican. It's positively ridiculous. However, you can learn Native American medicine from many excellent books depending on the tribe you wish to study. Any resource that doesn't claim a tribe and speaks generally is as good as garbage. Northern Shamanism and Tibetan Shamanism is an authentic thing and can be studied by anyone.
If you are looking for Cherokee medicine resources, I highly recommend the Garrett family books. They were given the blessing of the Cherokee Nation to write what they did.