I'm in act 2 and the Voices of Nerat gave me the quest to go to the library. This is the first quest he gave me. To open the last gate I need a spell that uses the stone sigil. No vendor sells the sigil not even the ones I recruited, and I don't have access to areas that should have it. I checked the wiki and such and it seems there's absolutely no way to get it because the location that has it will open only later in act 2. I did all the side quests, spoken to everyone, looted every possible location. What should I do now? Only thing I didn't do is that I didn't rercuit Sirin, but I doubt that will help. Any ideas?

Do you really need the sigil, though? I know the Sigil of Stone can be obtained from the Stone Sea, and even earlier if you side with the Disfavored. As for the Burning Library, have you collected all four incomplete passphrases to form the final clue? Try reading it if you haven't already, this might be the required trigger. If I recall correctly, you need only touch the sigils in the correct order (up, down, left, right), and not actually possess the sigils themselves.