Over on the Apple Discussions, there’s a quick hint to find and download the latest version of Safari. It seems that Apple are no longer offering standalone installers for Safari, instead telling people to get it from Software Update instead.
The command below uses grep to find the software update catalog url from the Software Update frameworks, passes this to curl to download and then greps the result for Safari.
You can then use curl again to download the .pkg installer for Safari.
curl $(strings /System/Library/PrivateFrameworks/SoftwareUpdate.framework/SoftwareUpdate | grep http://swscan) | grep Safari6