What are the core texts of this sampradaya ?
How many of these are surviving today?
Are there any digitial versions of the same ?
Lakulisha's Pashupata Sutra is the main text of Pashupata Shaivism according to the Wikipedia article on Pashupata Shaivism:
The Pasupata beliefs are summarized in the Pasaputa Sutra. This text was rediscovered in 1930 and is attributed to Lakulisa himself. Bhanmdarkar attributes the Atharva-Sirasto the Pasupatas. Also, in the text, their vow is recorded.