After Microsoft releases Windows 7 / Windows Server 2008 R2 Service Pack 1 to public, Microsoft now provides single evaluation media ISO for Windows Server 2008 R2 with Service Pack 1 for free download. To download the official ISO just access http://technet.microsoft.com/en-us/evalcenter/dd459137.aspx and logon with your windows live ID.
Or use this Direct Download Link
File Name: 7601.17514.101119-1850_x64fre_server_eval_en-us-GRMSXEVAL_EN_DVD.iso
File Size: 2.9GB
The expiration period expires after 180 days. After that the windows system will run with reduced functionalities and reboot every hour.