japanparts.com, have a hell of a lot of Legacy parts. Dave a STI Tuning can sort you out with a de-cat down pipe however from what i've heard you need a remap for any exhaust mods or you will get boost spikes which can damage your engine. Not sure that HKS kit is anything more than a fancy bit of aluminium pipe with a hks panel filter. Best off with a proper induction kit if you want to go down that route,again talk to STI Tuning i'd say