How did journal editors find peer reviewers before the internet? I personally find Google (& Web of Science and similar internet resources) such an invaluable tool that I can't imagine how editors from before the internet age did it. Presumably the only way is personal contacts, but one can only have so many personal contacts. There would also be no easy way to tell if a person is a suitable reviewer. Even if their papers can be found in the references of the article, they might still be a PhD student who's just starting out in the field.
Related: How do Journals find reviewers and their contact information? However, most of the methods described there rely on the internet.