Because of the privacy rules on when things like birth records are made public, I figured that I wouldn't be able to put in sources for my sister (who died when I was 9) beyond her birth record and her mention in Find A Grave.
However, one day, on a whim, I decided to look up her teacher, Lyn Hancock, who has written a number of books. Sure enough, Lyn has a web site, so I wrote and asked her if she remembered Fiona. She did, and was even willing to dig through her old files to find some of the stuff that Fiona wrote in her class.
So you never know where you're going to be able to turn up a source. (Or more information, for that matter.)