PLAN59.com is a gorgeous website dedicated to the commercial art of mid-century America. High-quality jpegs, gifs, tiffs and the occasional vector graphic. I mean, just look at how solid and chunky these images are. The whole thing is a sort of Jetsons meets High Society, with a bit of Breakfast at Tiffany's thrown in. Love it.
Oh, and be sure to sweep through their genius blog.