You can insert your install disk and install it from there.
If I'm wrong and there's not an option to install it like that, try to use pacifist to look it up yourself in the installer (preferably the disk/installer from your latest os).
I searched the MacOSXUpd.10.6.8.dmg but had no luck in finding the mail app so I downloaded the update .dmg which was the closest thing I could find (MailUpdate10.5.6.dmg) but gave me this error.
You can't use this version of the application Mail with this version of Mac OS X.
You have Mail 3.5.
Any ideas as to what is going on? Before all of this I deleted the application itself and it's ApplicationSupport folder.