How can I find out more about classical nondual Tantra?

You can start with these resources:

Dr. Christopher Wallis’s website and YouTube channel:
Christopher Wallis, also known as Hareesh, is a Sanskritist and scholar-practitioner of Classical Tantra with thirty years of experience.  Hareesh teaches nondual Śaiva-Śakta Tantra.

Hareesh’s book Tantra Illuminated is an excellent guide for the beginner.

Dr. Kavitha Chinnaiyan’s website and YouTube channel:

Kavitha Chinnaiyan is a practicing cardiologist with a remarkable breadth of expertise in medicine, technology, writing, mysticism, art, music.  She teaches in the Śrī Vidyā lineage.

Dr. Sthaneshwar Timalsina’s website and YouTube channel:

Ācārya Dr. Sthaneshwar Timalsina is from Nepal. He was traditionally trained, consecrated with sarvādhikāra-sarvasāmrājya dīkṣā and is authorized as Śaivācārya within Nepali Sarvāmnāya Order by his dīkṣā gurus.  Highly recommended once you have some basic knowledge.