SQL Server 2008 Service Pack 4 and SQL Server 2008 R2 Service Pack 3 were the last service packs for both versions respectively and were originally released in Microsoft Download Center in Sept 2014. Due to popular demand we have also made them available on Microsoft Update Catalog. You can download them from the catalog by adding the packages to the basket.
UPDATE (July 01 2016): The packages are also available in the WSUS catalog.
Ajay Jagannathan (@ajayMSFT)
Principal Program Manager