I doubt you'll find anyone selling a Windows 2000 CD without also selling the license...in fact, I doubt it's even legal to sell them apart.
Regarding upgrades, you can upgrade Windows XP without needing an OS installed first, but you need a CD from a previous version of Windows...which I guess you don't have. Although how did you get XP Pro Upgrade installed in the first place? If memory serves you can use a Windows 98 CD.
What are you going to do with computers that old anyway?
