So, after the 2007 model-year of WRX/STI, Subaru stopped offering paper service manuals. Instead, they offer them online (for a fee). A 3-day subscription will give you access to the online database of manuals. You can download the PDFs for later use.
http://techinfo.subaru.com/index.html
Some forums also have PDF copies of the service manuals. IWSTI, for example, has them in their Donating Member section. Also, a quick scour of the web (via Google) may get you some hits.