Where I can get a licensed copy of Windows 2000 with SP2 or earlier SP edition

I have a old network that’s currently “certified” with Windows 2000 and Sp2. My client is just adding two more computers and I have to downgrade to match. Upgrading all the current computers is not an option unfortunately.
I am desparately looking for the Windows 2000 with Sp2 or earlier edition. If you know of a place where I can purchase this product, please let me know ASAP.

Thanks,

Kevin Ngo