Hi mate welcome to S.O.C
You have to look for it on the Gearbox itself from memory it's passanger side of the bellhousing. If you shine a torch down the left side of the motor at the back near the firewall where the gearbox meets the motor you should see a white sticker with the code on it. If not try the driver side, should be able to see it.
but some links here that might help too
http://spda-online.ca/content.php?44-transmission-chart
http://www.catherineandken.co.uk/sti/trans.html
http://subarugears.narod.ru/Index.htm
it's all the same info just gave multiple links just in case anyone is looking for them at a later date and the site is down. Forward planning :D