These digital galleries have so much visual reference and research; I highly recommend taking the time to look through it all and bookmarking or taking note of what you might find useful in the future.
John sent me a link to his Props on Paper site. John is the head of props at the Grand Theatre in London, Ontario. He has a large number of paper props he has produced from over a dozen shows.
There are a number of other sites which have collections of downloadable paper props. You don’t necessarily need to search for theatre props; there are large communities of replica prop makers and role-playing game prop makers with all manner of documents available online.
A great source for researching vintage ephemera is eBay. Though it can be hit or miss, it’s one of the few sites where hundreds of people are uploading photographs of actual historic relics. You can search for your specific prop need, or search for “vintage ephemera“, or browse the Collectibles category.
You can also find a large number of actual historical documents online, which you can adapt for your use. For example, say you’re dressing a New York City apartment building, and want some realistic documents hanging in the lobby.