If at all possible I would stick with the original exhaust system if you want the best economy and power.
Check the Buying and selling section.I'm sure there must be a USA supplier that has them but both Wemoto and David Silvers over here in the UK carry them and are reliable and ship to the USA.
I think the CX500D are the same as the standard CX500s and it's only the,"Customs/Turbos and Euros" that are different but best check with supplier,
If you give the year,engine size make and model to them they will know,
http://www.wemoto.com/bikes/
http://www.davidsilverspares.co.uk/
HTH