How to obtain service packs for SQL Server 2008 R2?
Where to find the information about service packs already released or planned for SQL Server 2008 R2?
SQL Update information, for all versions, all types (Service packs and the monthly cumulative updates) is published at...
I believe that only Cumulative Updates are released for the SQL 2008 R2. The current SQL Server 2008 SP is SP2, but it does not apply to R2.
R2 is a separate release.
If you go towards the bottom of this KB article it is updated with the latest SP/CU for each version of SQL Server as it is released. It doesn't give a link to the KB but it is a good central point to check.
New: Microsoft has created an update center that will show the latest SP/CU currently released with a link. I'm not sure how quickly they will update this though, example being that they don't show SP1 for SQL 2012 on there yet.
I'd try the Internet. Surely this is a simple thing to google?