Where do you retouchers get your backgrounds / digital props?  Stock sites?  Can you get a whole library of these elements?

www.sxc.hu is the biggest archive I know with free images. Check the license for each image though, as some images needs approval from the photographer.

www.everystockimage.com is a meta search engine that searches many free sites for images.

Appreciate that!

The mother of stock sites http://www.bluevertigo.com.ar

Lots of stuff http://browse.deviantart.com/resources/