PicFinder is a search engine for free stock photography that you can use commercially. It searches sites such as
Image:After ,
MorgueFile and the great
SXC.hu, making it that little bit easier and quicker to find images.
It also returns results for paid stock too.
MagazineArt.org is a huge database of magazine covers from the 19th and 20th century. It takes a bit of rooting around to find stuff, but there are tons of lovely covers for inspiration.