If you can't find a trained guide, having close trusted friends sit for you can be of tremendous therapeutic and healing value. And, yes, working with a trained guide can be of huge benefit -- they support you in creating a safe and powerful set and setting. The work can definitely be shamanic - one of the things these substances have the potential to do is to teach us how to shamanically journey within ourselves. The safer our psyche feels, the deeper we can go.
I want to mention that there are many professions other than psychologist that are a natural fit for safely guiding people in states of expanded consciousness.
Many people who provide this kind of service are trained and apprenticed specifically in this kind of work after education and work experience in other healing and service oriented fields like therapy, counseling, ministry, medicine, nursing, bodywork, music therapy, etc. And they have lots of personal experience being in expanded states themselves, working with a guide (that's part of the apprenticeship).