Are you looking for a used hard drive or a new one? If you're looking for a new HD, then you're probably better off buying it online as the margins are too small to make it profitable for anyone to sell it to you and still make money. If you're interested in a used HD, then you might find some sellers. Western Digital 500 GB ($99) http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=2795126&Sku=TSD-500AAKS
I think http://www.newegg.com has some really good deals also,but I would recommend one of the Seagate Barracuda drives.I have three of them and they are real quiet and very reliable.I also have a 5gb usb drive from Seagate that works wonders as well.Here is a link to a 500gb Seagate Barracuda http://www.newegg.com/Product/Product.aspx?Item=N82E16822148136 .It is an internal drive though,if your looking for an external I would recommend one of the Seagate FreeAgent Desktop which you can find here http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=3094187&CatId=8 .I like Western Digital as well,but I think Seagate harddrives are the best.