That's classic shamanism, core shamanism allows one to be a shaman and not be called by the spirits to do so. Also, anyone who isn't specifically of the Tungus region (where the term shaman comes from) they aren't really a shaman; they are a shamanic figure with their own culturally relevant term. For those people who do not have a culturally revelant term that they know (such as within the Northern Tradition), shaman is an acceptable term. That still doesn't necessarily make them a shaman, they are just using the term as best as they can.
So you can study/learn it from either the spirits or people, but I would first learn what it means to be a shaman, what sorts of challenges you'll face due to it. Kaldera's shamanism series has a pretty awesome chapter in there that tells you why you wouldn't want to be a shaman. I think you might want to read that before jumping into things. Just be careful what you wish for/call yourself, because someone might take you up on it.
AP and the third eye (or brow chakra) are used within and come from specific cultures. The biggest culture this has at the current time is within New Age dogma. New Age dogma states that everyone, regardless of what their path is, must learn and work with their chakras otherwise they can't function properly. The chakras come from a specific culture which the New Age movement has picked and chosen from. They come from Hinduism. Karma (another part of New Age dogma) also comes from Hinduism, however karma is limited to their culture, to their believers. To anyone outside of it, karma doesn't apply. So the chakra system would logically be the same way.
This concept of things only applying to the people within the tradition is something seen with very old cultural traditions and within a select few newer ones based upon older cultural traditions.
But I digress, my point is the third eye isn't a requirement for seeing spirits while on the physical plane. As for AP, many people do not use the terms AP in reference to journeying into the Unseen or Spirit worlds, because a) it is not culturally relevant or b) AP and what they do sound nothing alike. I'm one of the people who don't use this term because whenever I leave my body it seems much more colorful and less controlled than AP is.
So again, you don't need to know AP to see spirits.