You can download Cumulative update package 5 for SQL Server 2008 R2 Service Pack 1 using the below link:
Cumulative update package 5 for SQL Server 2008 R2 Service Pack 1
sql server 2008 r2 cumulative update list given below:
Other Cumulative update packages download links for SQL Server 2008 R2 Service Pack 1:
Cumulative update package 6 for SQL Server 2008 R2 Service Pack 1
Cumulative update package 4 for SQL Server 2008 R2 Service Pack 1
Cumulative update package 3 for SQL Server 2008 R2 Service Pack 1
Cumulative update package 2 for SQL Server 2008 R2 Service Pack 1
Cumulative update package 1 for SQL Server 2008 R2 Service Pack 1