For starters go to
www.flowmastermufflers.com or
www.summitracing.com. They both list Flowmaster Force II catback for 3.8 V6. There is a split pipe just behind the cat that goes into 2 Flowmaster mufflers and dual OEM style tailpipes with stainless tips. You will have to get a hanger for the right side muffler and tail pipe.
Summit has the best price.
Check the prices at some of the Mustang Parts stores too, sometimes you can find a better price on the same system by shopping around. Summit or Jegs may be the best though, because they charge 8.95 for handling and they ship UPS ground for FREE!