Where to download Sql Server 2008 R2?
If you want it for a lab environment you can get a preconfigured VHD with an evaluation edition of 2008 R2 installed. It is valid for 180 days:

SQL Server 2008 R2 Standard on Windows Server 2008 R2 SP1 Standard (x64)

Also, you can get it if you or one of your colleagues have a MSDN subscription. If the subscription is not yours you can download it from there and use it for the 180 day evaluation period.