any muffler shop should be able to do it.....is it the gasket between the intermediate pipe and the manifold on the passenger side? if soo u can actually make ur own out of copper as long as u anneal it(get the peice red hot and quench it in used motor oil and it will be soft enough to form when u cut it out)
give it a good coating of the copper rtv sealent and it wont ever give u probs again!