Try the megan racing one for the is 300. Only needs a hole drilled and a nut welded on for the lambda sensor.
Looks great and fits great, plus it gives some nice extra oomph. Had it on my previous is and the only disadvantage was that people in the back complained it was a tad loud.
http://cgi.ebay.com/ebaymotors/Lexus-IS300...sQ5fAccessories