Not to give you a hard time, but shouldn't you have asked those questions before you signed on the dotted line and drove away from the dealership?
Since you say the the car is a Certified Pre-Owned, one could make the assumption that you bought the car at a Nissan dealer. Is this correct?
I would suggest that you ask the dealer that you purchased the car from if something like that will be covered by warranty. The dealer's opinion trumps board member's opinions.
Here are tips on how to get an extra key fob. The dealer you bought it from should be able to help you, ask them. If they can't, try this:
http://lmgtfy.com/?q=2008+Nissan+Altima+Keyfob