You can try here but more than likely need to contact the seller, a licensing specialist or here.
https://www.microsoft.com/Licensing/servicecenter/default.aspx
https://support.microsoft.com/en-us/kb/326246
Regards, Dave Patrick ....
Microsoft Certified Professional
Microsoft MVP [Windows Server] Datacenter Management
Disclaimer: This posting is provided "AS IS" with no warranties or guarantees, and confers no rights.