In the past, it was the newspaper morgue, old diaries, and, if possible, a company historian. Trivia books picked up a lot of things as well.
I did a trivia column for a humor news letter because I got tired of getting forwarded emails full of BS trivia. Did you know a duck's quack does not echo and nobody knows why? Cecil Adams got that bit of trivia and a duck and after testing, determined that a ducks quack does echo and people are really gullible.