Pricewatch is a great place to find prices, but be sure to check the company's reseller ratings to make sure they are legit. It seems for every decent company PriceWatch searches, there are 3 fly-by-night operations. Be careful.
Also, NewEgg.com is the best (IMHO) computer parts store online. I think just about everyone knows about them, but they are worth mentioning anyway.
ZipZoomFly.com is also a very great place for computer parts, although neither them or NewEgg will have really old parts of course.
Otherwise, there's always eBay.