Depends on the computer. I have an ancient Packard Bell as well as this system, so i have been known to peruse PC Liquidator as wel as a few other salvage sites.
I have NewEgg, ZipZoomFly, TigerDirect, ThinkGeek, Retrobox, TechForLess, and Comp-Recycle (all dot com) in the 'Tech' folder of my bookmarks.