As the title states, I would like to buy a 350z to g35 exhaust extension. Must be 3" diameter, not 2.5".

New or used, doesn't matter.

 

Any muffler shop can make you an extension and it will be cheaper than buying one that may or may not fit plus the cost of shipping!
Gary

 

are you suggesting there is a lot of variability between the extension lengths necessary? From my searching I found 8" to be the the normal extension piece length.

 

I think the point is there's no "special" pipe extension. If from your research 8" of pipe is all you need have a muffler shop weld some flanges onto 8" of stainless and call it a day.

 

My second exhaust system was a 3" true dual experimental model from Magnflow designed for the 350Z back in '04, my only cost was having the extension made plus install at my muffler shop. That's the right way when you want everything to fit and work together...no leaks or issues!
Gary