Missing Windows Installer cache files prevent SQL Server 2008 R2 Service Pack from installing

When trying to install a SQL Server 2008 & 2008 R2 Service Pack you may see the following error:

1612 The installation source for this product is not 
available. Verify that the source exists and that you can 
access it.

These issues can occur when the Windows Installer database files (.msi) or the Windows Installer patch files (.msp) are missing from the Windows Installer cache, which is located in %windir%\installer.

To resolve this issue go to KB969052 and run the FindSQLInstallsOnly.vbs from a command prompt. The output file sql_install_details.txt will contain information on the missing files and will advise where to copy these missing files to. Follow the instructions and once all the missing files have been replaced re-run the script and the output file should confirm the existance of the missing file(s), it should be similar to the below:

SQLServerXXXXXX.msi exists on the LastUsedSource path, 
no actions needed.

After you have confirmed there are no more missing files re-attempt the SQL Server Service Pack installation and this time it should be successfull. I have seen this issue a handful of times in the past and it is worth noting KB969052 so that you can respond swiftly, especially if you are running according to a tight maintenance window.