For OEM parts, you can try one of the various online discount parts
sites, Jerry Rome Nissan Parts (jrnissanparts.com), Courtesy Nissan
(courtesyparts.com), or Midway Nissan (midwaypartsteam.com).  I don't
think you'll be able to just buy the OEM inserts - you might have to
buy the entire mount.  If you want something firmer, you can buy Energy
Suspension mounts (search for online vendors) - I have read that people
have had more vibrations using the performance oriented polyurethane
mounts.

Also if you have the GLE trim level, one of the mount has
electronically controlled firmness - this will not be available with
aftermarket inserts.

Cheers.
Nirav
96 Max GLE, 117k