Where can I find the earliest possible version of the Mahā Karuṇā Dhāraṇī ("Great Compassion Mantra")?

I am looking for the earliest possible version of the Mahā Karuṇā Dhāraṇī ("Great Compassion Mantra") in Sanskrit or Pali as part of tracing it's origins. My hope is to find the Chinese contributions. I suspect it can be found in a book, either in print or not, but entirely in Sanskrit. As I am not literate in this language it hinders my task. Can anyone recommend where I can find such a book or source containing the text of the Mahā Karuṇā Dhāraṇī ("Great Compassion Mantra") dating before the first Chinese translation of 627 AD? Is there a library that one could recommend that would have a good chance of having it? Thank you so much for any help you can give.