How much do you want to spend? What type of sound are you looking for Quality or Quantity? You can mount a sub in the factory sub location either vie free air type sub or build a box in that location (much more difficult but certainly possible) as for amps they can be mounted in the trunk or in a custom amp rack. For places to shop I have had good experiences at thezeb.com. There is also Crutchfield and http://www.edesignaudio.com has some great stuff as well as Etronics.com, sounddomain, and on on. I would recomend you bring you favorite music to your local stereo shop and take listen.
A forum community dedicated to Nissan 350Z / 370Z and Infiniti G35 / G37 owners and enthusiasts. Come join the discussion about performance, modifications, troubleshooting, exhaust, turbos, maintenance, and more!