Parts (mostly used) are always available on Ebay. In addition to
www.subaruparts.com you can check
www.1stsubaruparts.com.
Members on here sell various used parts, trim parts and a few aftermarket parts.
You can have an exhaust made or you can check out Stebro for a very nice sounding and looking system even though a bit pricey, but does have some performance gains. Also, if you read on here you can see where there are potential problems with trying to change the exhaust without losing performance.
Lee