Lost Windows Server 2008 R2 disk

Where can I find the DVD/ISO image of the OS that came with the server. We cannot seem to find the original media that (probably) came with the PowerEdge server.

Server: PowerEdge R415

OS: Windows Server 2008 R2

I have the image of the license keys. Need only the media. Apparently the key works only with the OEM version of the software i.e. from DELL only.

Thanks.