I'm always on the lookout for photo and illustration sources for work or personal use. When I saw this wall art idea in the April issue of Martha Stewart Living, I followed the source of the balloon illustrations back to the Library of Congress's prints and photographs catalog. It's a gallery of digitized images you can download and print, sorted by collection. All the images I looked at had "no known restrictions on publication," and in most cases, you can grab high res files ranging from 3"x5" to 11"x16" at 300 dpi.
The hot air balloons on the clipboards are from this collection. I also loved the WPA poster collection. Enjoy!