I found a number on eBay;
http://www.ebay.com/sch/i.html?_kw=2003&_kw=office&_kw=professional
But I wonder if this is worth a phone call to Microsoft to see if they will send you a replacement CD? Since you have the key they might do that.
Mark
I need to reload office on a Xp machine and opened the disk protector only to fine a crack from the hub to about 50% if the data tracks. The key is valid, is there any way to obtain a loadable disk and still use my key?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic