Is the December 18, 1857 morning edition of [[The New York Times]] an acceptable source? It's rather hard to find a copy of it these days.
Probably. I would imagine a big library will have a copy on microfiche.
How about the [[ISO 9000]] standard? The International Standards Organization charges thousands of dollars for access to it.
Having to pay large amounts of money probably makes it unacceptable. Expensive scientific journals are ok, as those can be found for free in a library. If memory serves, the ISO doesn't allow libraries, etc., to make things like ISO 9000 publicly available.