If you are looking for something descent quality, find yourself a custom exhaust shop and let them do it. They should have a proper size hi-po muffler in stock. Most larger towns have a guy that is well known amongst Mustang and Camaro driver.
Otherwise, meinike and those types of places could swap out rotted muffs.
If you absolutely cannot wait, you could have a Dynomax cat-back shipped to your door for $343 from Summit with overnight delivery.