I love to get pictures and textures from stock photo sites. I have used Stock Exchange and Image After, but I just found this huge list of links.
You need to be aware that some of the sites are free and some are not. Also, some of the photos need to be linked back or some can not be altered, but also . . . some state that you are allowed to use their photo in any way you please. Just do your reading on each site.
http://www.bluevertigo.com.ar/bluevertigo.htm
Last night I needed a tire tread for a racing layout so I found a picture and turned the tread into a PS7 brush . . . COOL !
Sallie