I've always had off-road exaust on mine as well. I stripped my cats the first week but, then I really disliked the sound. It really sounded better with the cats, x-pipe and FR exaust. I bought long tubes, h-pipe and some M80s and i'm currently playing with different mixes of components.
I mentioned Cats because, if you still had them, that would be the cheapest way to quiet things down and you will only lose 1-2hp.
Craigslist is your best bet but, Ebay has quite a few stock muflers if that's what you are looking for. you are going to have to pay shipping to most people on here anyways.
2006 Mustang GT
Pacesetter Ceramic L/T's
FR hot rod cams
FR Intake manifold
Hurst Competition Plus Short throw.
2001 Mustang GT (miss this one)