If you are looking for the classical Tantric text, use these public domain libraries:
Search for "Brahma Chinta" here. This digital library hosts millions of old books that have entered the public domain. You can usually find scanned copies of older editions in Hindi or Sanskrit. brahma chinta book pdf
The largest repository of Swami Sivananda’s works. Check the "Free E-Books" section. If Brahma Chinta is an original DLS publication, they often offer it for free as a service to humanity (Seva). If you are looking for the classical Tantric
If you want, I can: