Most of the model specific nature of an exhaust system is not the muffler itself but the piping and how it attaches to the underneath of the car.
It has to be sized and bent properly to work its way through from the front to back.
If you are just wanting to replace the muffler and tip, you should be able to purchase those parts individually and get a shop to cut off the old and weld on the new for not that bad of a price.
If you are wanting to do a full "cat back" (everything from the cat to the tip) then a custom one will cost more as the pipes will have to be custom fit.
One downfall to this is you won't know what kind of performance effect it will have on the car as a single muffler will act differently on different model vehicles compared to their OEMs.