Sure you can search Google images and come up with great results. But check to see the copyright status, it is unlawful to use copyrighted images even in collages.
Go directly to a site full of copyright free or public domain images. One of the best places to look is Flickr. Museums and archives are putting up historical images and photographs that you can use, and copyright status is easy to check. The most recent addition to “the commons” is the National Museum of Denmark, the source of the image above.
The mariachi photo above is also from Flickr’s commons. It is one of New York Public Library’s photos.
But you can find much more than photos.
I saved the best for last – there are a ton of vintage images ripe for plucking. The vintage Halloween card above was also made available from the New York Public Library. Go take a look.