The word “Guru” means “dispeller of darkness”, and really refers to the Infinite Consciousness acting as teacher and guide to individual souls. So since Infinite Consciousness is omnipresent, the real Guru is within us already.
When an individual has attained self-realization, they are often referred to as a Guru, because the Infinite Consciousness within them is able to act and speak without the distortions of ego. So they are able to play the role of a perfect teacher and guide to others.
In the Bhagavad Giita, Arjuna asked his Guru, Krisna, whether it was possible to attain enlightenment through the guidance of the Divine, inner Guru, without the assistance of a Guru in physical form. Krsna told him that it is not essential to have a physical Guru, but if you do not, it will probably take you about 10,000 times as long to attain enlightenment.