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.
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.