How did researchers find articles before the Internet and the computer era?
We depended on libraries and librarians. Grad students would spend hours in, say, the math section of a good academic library, going from book to book and taking copious notes (on paper, of course). 

But, often enough, the next paper we needed to look at wasn't in that library at all, so you would go to the librarian and ask for a loan of the resource from another library. 

But it was also an important technique to use the librarian as a 'knowledge expert' who could, and would, suggest things for you to look at and places to look. I don't know if librarians still get the training to do that. 

And, of course, you could ask your colleagues for hints about which rocks to uncover to find the gems.