There are a couple of ways to go about obtaining media. In some cases, it may be possible to simply download and install a trial version of the OS. Generally, you can use your purchased license key to activate a trial version of Windows, which converts it into a full version.
All the evaluation versions of Windows products can be downloaded at https://www.microsoft.com/evalcenter/
To activate the trial version with your license, please refer to the activation method proper to your Windows Server version. It is also often possible to extend the Server Evaluation Period.
If this is not an option, go to the Microsoft OEM System Builder website and fill out the Media Replacement for End Users of System Builders form to order a copy of your media. For more information, links and references or keys, refer to Microsoft Article ID# 326246 - How to replace Microsoft software or hardware, order service packs, and replace product manuals.
You should have the following information handy:
Other helpful links on Product Activation and Licensing from Microsoft can be found on the following link: