I suppose that gettyimages.com is one of the best as well as just googling something. It really depends on what is asked because some clients have their own specific resources laid out for you that they want you to find already. National Geographic has a good website for worldly imagery and I guess you can find some things on youtube quite readily as well. I have a solid DVD collection with a lot of good reference material in there such as the Planet Earth DVD set.
I do not have "fixed" places. I surf through the internet. And encounter things.
Sometimes I do save their references, sometimes I don't. But I believe in movement, not in fast-and-good recipes.
I spend a lot of time at the Book Cover Archive: http://www.bookcoverarchive.com and various design studio sites. Also Galleycat, Unbeige, and many, many book design blogs.
delicious holds all my links... a few:
None in particular, though I'm addicted to the following design showcase portals:
I normally follow whatever hot links they have to brushes and textures, etc, as well as looking at all the new work everyone is doing.
adobe, that is really it...some random apparel forum
best designs.. I am always online...and does a lot of reserch by community, culture, new trends etc..
Many, like Esquire magazine, GQ magazine, FHM, MAxim, Playboy. Mostly for lifestyle magazine.
google, deviantart, shutter stock
Photo stocks, textures and vectors.
Books; such as Amazon.com.
CGTalk, CGChannel, High end 3d, google, utube, various places wherever art and technology pop up.