Where can I get the pipes to dual out my exhaust on 99 Grand Marquis?

[Nate]

For some reason only some Grand Marquis got duals. Optional I guess. I know Mustangs use H-pipes and X-pipes, is there something like that for me? The cars do share the 4.6L V8, maybe pipes?
Two flowmasters, and performance cats await.

[mastertech35]

Any muffler shop. You are going to have to buy a special muffler.