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.