Creative works prior to 1924 are public domain in the US, and there are many newspapers still around that are older than this. I’m not a lawyer but theoretically we should be able to freely use pre-1924 articles and, unlike Wikipedia, potentially use every single sentence in an article.
Is there any legal reason why these couldn’t be used? @nukeador
Some sources like http://www.newspapers.com are behind paywalls but the New York Times archive is pretty open and the restrictions are not unreasonable. Also the Library of Congress has newspaper archives (although their site is slow and times out a lot).
Wikipedia also has a big list of archives: https://en.wikipedia.org/wiki/Wikipedia:List_of_online_newspaper_archives#United_States