Not everyone is a restoration purist. They will paint when a room was wallpapered or stenciled, but for those that want to do an "over the top" restoration there is a resource to be able to use actual wallpapers from the era.
Secondhand Rose is one of the largest dealers of "vintage papers". Original Papers that come from a variety of resources. Their inventory changes regularly but if you are diligent you may just find that perfect paper for that room that you have been meaning to get around to.
Their website is at http://www.secondhandrose.com/
They stock a variety of papers from various periods so if you house is built in 1900 or 1960 you probably will find something interesting.
Need something interesting for your floors? They also sell original linoleum's.
Other resources? Try Ebay or even Craigslist. I've even found vintage wallapapers at yard sales!