Custom, there is no manufactured dual exhaust for our cars.

Any exhaust shop can make you a nice setup.

C55 exhaust would be one option. Modified to match.

Also, many examples around of "fake" quads.

I recall reading its a major transplant - c55 boot/trunk floor insert.
The only quad for the w203 is the extension of one of the muffler ports to the other side and splitting to tips as far as I know.
The Germans have them on ebay(kits) or would be custom.

I have a modified one on mine with MH Dezent exhaust tips

Going to the quads is much more difficult than one may think. I did a lot of research when I had the C230 and eventually gave up because it would have been way too much work.

The spare tire well on the C55 is different (and smaller) so it sits in the center between the two mufflers. The C230 is larger because it gives you a full size spare and it takes up most of the right side because there is no right side muffler.

I am guessing most of the Benzes with single exhaust probably don't have room for another muffler on the other side. So unless you are doing what Macbryanie said, you won't really have a true dual muffler quad setup. The additional pipe out of the muffler port is purely to have a quad look.

All true unless you install a muffler where the x pipe would usually go and just install the resonators/silencers in front of the end tips.
I got lucky when I picked up the C55 spare tire well from a local member for almost nothing. It had a 2 inch corner chipped during shipping. The vendor sent him a new one and told him to keep the "damaged" one.

I have a quad setup on my c32, had to cut the trunk.

I put in an aluminium rear end instead, since the wheel well doesn't serve squat.

Heat mat underneanth and dynamat throughout the boot and new floor.