I have the rear exhaust pipe from my 2007 WRX that is of no use to me anymore. It has covered 35k km and is in good condition.
Collect from Lund for 250 SEK. It is not worth posting it as postage will cost more than the exhaust itself!
It may fit a Sti but you have to check.
It is the rear exhaust pipe with resonator in this diagram: http://wrx.grapon.com/techdocs/wrx-intake-exhaust-diagram.gif