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.
i dunno why people dislike thomson drives so much. Mine reads every brand of dvr/+r i throw at it.
It just doesnt read cdrs, but who needs those anyway