depends wut kind of things you want.. overall i'd suggest googling ur topic, and going to some of the webpages u find. if you're looking for stars and stuff, try looking for a fanpage with a gallery for them...... overall i find that if you take a 'desktop wallpaper' similar to what u want, then manipulate it, it makes a great background..
some good stockphotos:
http://www.powerpointart.com/