What kind of interior? Stock? Other models? UKDM/JDM?
Ebay is a good place to start but locally is the best since shipping of large interior piece gets a little spendy if you're not prepared. If you're looking for new seats check out nasioc for people selling wrx/STi seats. If you want the UKDM/JDM seats then check vendors on this forum and ebay.co.uk.