Check out Boxer4racing.com for parts. I know Borla makes a header for the 2.5 litre engine and Stromung makes a nice stainless catback exaust. Random Tech makes high flow cats for the 2.5 litre.

I'm sure that you can find KYB AGX or GR2 struts for the Legacy and Eibach or H&R make springs that fit the Legacy as well.

Check out SPO Motorsports for f&r swaybars and you can use whitelines endlinks.

Check out Cobb Tuning or Ganzflow for cold air intakes

Hope this helps