how much of it do you need? if it's just a small piece, go to an exhaust shop and have them fab something up. if you need that whole half of the exhaust you might as well buy a new one. check ebay, theres a dude selling flowmaster exhausts pretty cheap
eBay's probably your best source for the right half too, if you do need more than a small piece, but jadedinpa hit it on the head - exhaust shops have many wonderful things... (we actually made a ram air intake at an exhaust shop once - no it wasn't on a mustang! We put it on an escort )