There are various sources where you can find photographs that you can use, either because the photographer has granted permission for this, or because they're copyright free.
Another good source of photos is Flickr, but be sure to use the Search Tool that enables you to find those photos labeled with a Creative Commons Attribution License. This license allows for copies and derivatives to be made from a photo (which a painting would be) and commercial use (which you'd be doing if you then sold the painting or exhibited it in a show) provided you give credit to the photographer. To check what copyright applies to a particular photo in Flickr, look under "Additional Information" in the column to the right of a photo, and click on the tiny CC logo to check the Creative Commons License.
Then there's the Public Image Reference Archive Morgue File, which provides "free image reference material for use in all creative pursuits". And Stock.Xchng where some photos can be downloaded for free. When you do a search, the free photos are listed first, and the ones you'd need to pay for below a bar saying "premium results".
Artist Jim Meaders says he uses eBay as a source for finding old black-and-white and sometimes color photos and that this can provide very interesting subject matter. He says: "Almost all of the photos I've bought are snapshots by individuals. I find the fact that they're black and white to be a positive thing because it allows me to create whatever colors I want in my paintings (even abstract colors) without being influenced by the colors in color photos."