I was searching for a cold air intake for a louder sound but I want to keep the stock sound. I saw on youtube a lot of g35s with borla,injen cold air intakes and exhaust and sounded like civics. Someone told me to get the nismo cold air intake but they are hard to find . Does somebady know a good way to increase the stock sound maybe straight pipe or a cold air intake that keeps the same sound ?

 

Get a ztube(either OEM 350z or stillen )with a drop in K&N air filter(google search it).It will still keep the same tone it'll be just a bit louder I assume.The stock intake on the G restricts air from what I've heard.
EDIT:I found a video relevant to your question.

 

I was in the same boat as you. After listening to lots of exhausts, I ended up going with the Nimso exhaust system. It still sounds like a G but deeper and with a growl. I took off my popcharger because it made my car too loud but I put it back on last weekend for S&G's.

I have not installed my Nismo CAI yet because it seems like a lot of work, but if I do, I will post a video of it with my Nimso exhaust.

 

There is no advantage to installing a z-tube or intake other than sound! If you love how your coupe sounds leave it alone. If you want it slightly louder, get a Stillen Z-Tube and K&N drop in filter....not a lot of noise but a different tone. Exhaust systems, they all make more noise than OE! What you need to do is attend a few meets this year and hear different systems in person, that's always better than recorded sound!
Gary