Plan 59 is an online museum and gift shop of mid-century illustrations and art. Full of vintage advertising images, sales literature, and just plain old retro art, it's the kind of place in which you can get lost for hours. They even have a collection entitled "Decor" where you can resource and get inspiration for your own mid-century spaces.
Many of the images are also available as a digital print. Head to Plan59 to learn more.