You'll have no problem buying a replacement hdd from newegg for the laptop, your laptop requires a ata5/6 hdd, You are correct in that a replacement drive from dell would be more expensive, its not nessessary to get them from dell. Just remember to remove the adaptor from the old hdd (there is a pin adaptor on the old hdd than must be put on the new hdd, carefull of the orientation and not to bend any pins) before you put it into the hdd caddy (the metal shroud around the hdd) Techpro is correct that laptops are generally much more finicky to component swaps than desktop computers, luckily laptop hdd swaps are easy and not a problem.....
http://www.newegg.com/Product/Produc...82E16822136130
or a
http://www.newegg.com/Product/Produc...82E16822146230