The general consensus appears to be that a living guru who is accessible and can give initiation to the seeker is absolutely essential for any form of progress in Sadhanas.
However, in his book "The Ancient Science of Mantras: Wisdom of the Sages", Om Swami gives the following mantra for people without a human guru:
gururbrahmā gururviṣṇurgururdevo maheśvaraḥ, gurureva paraṃ brahma tasmai śrīgurave namaḥ.
This blog post claims that people without a guru can invoke Lord Shiva as a guru with the following mantra:
ॐ हंसः शिवः। सोऽहं हंसः। शिवः हंसः। शिवः सोऽहं।
On the same website, this other blog post offers the following guru Gayatri for people without a guru:
ॐ गुरुदेवाय विद्महे परमगुरवे छ धीमहि तन्नो पुरुषः प्रछोदयात्
Is there any scriptural basis for any of the above claims?