There are many sources for vintage images, both free and paid for, out there on the internet, but a lot of these images are either of poor quality or extremely expensive, and often come with onerous conditions attached.
The only source that we know of for truly high-quality vintage art, suitable for any application and available for immediate download at a reasonable cost, is Mindy Sommers' Vintage Art Download. Because Mindy creates the majority of her images at a resolution of between 300 and 1200 dpi (some at up to 2400 dpi), you will be able to produce a much larger print with no loss of clarity. Her site is also remarkable for the fact that she imposes absolutely no restrictions on the uses that you make of the images after purchase.
We also love Mindy's own original artwork at Color Bakery. She is clearly a talented artist in her own right.
We would also recommend DigitalCollageSheets.com, where Michelle offers a good range of vintage images for the mixed-media artist, again available for immediate download at a reasonable cost. She also offers a couple of free collage sheets. All of her images are at a resolution of 300 dpi minimum, and made available subject only to the condition that they are not resold either individually or collectively as part of any other collage sheet. You should note, however, that the downloaded images have to be printed 'as is', and cannot be modified using your graphics software.