If you need to buy a replacement keyless entry car remote for a 2002, 2003, 2004, 2005 or 2006 Nissan Altima or Maxima, the correct transmitter is Remotes Unlimited part number 504-1504. This 4-button keyfob had several Nissan part numbers at different times. The most common FCC ID that appeared on this remote is KBRASTU15, though there are others.
The keyless entry systems that use this remote are all user programmable. You can buy this remote at most Advance Auto and Pep Boys stores, where it is stocked as part CPR-8504.