Links to the TMG 9 installers are shown in my article on the Future for TMG Users.
If your old computer is still operable, you can recover the serial number from it. If not, you need to find the email from Wholly Genes that contains it. If that fails, at last report emails to firstname.lastname@example.org requesting old serial numbers were still being responded to, but you must be patient waiting for a reply.
If nothing else, you can buy a new serial number from Harry Goegebeur whose address appears as a link in my article.
Once you get back up running make sure have copies of the installer and serial numbers in a safe place as recommended in my article.