The Xbox requires specially built/modified DVD drives, so it's not like you can buy one off the shelf somewhere. Local modders may have some extra from scrapped/modded Xboxen, check on
www.xbox-scene.com. Avoid the Thompson drives, they are evil. Philips and Samsung (preferred) are the drives you want. The Samsung drives tend to be expensive, though. Another option is the Samsung 616T, a non-Xbox drive which can be modified to run on it. You can find one pre-modded or you can mod it yourself -- use ebay to get it and Google to find a modding guide.