Hmm, sounds like you're talking about your rear subframe bushings. There are 4 big bushings that hold the rear subframe to the car. When these get old, they let the subframe move around and smack up against the unibody. These are the Nismo bushings you are looking for.
http://www.courtesyparts.com/M...00001
However, there are several companies that sell metal inserts that basically lock the subframe in place and completely take the bushings out of the equation. The only drawback is a little more noise and vibration coming into the car from the rear end, but they are a lot cheaper, and don't require the removal of the subframe. If you can live with a little noise and vibration, you should look into some subframe inserts.
These guys seem to have the cheapest ones around.
http://www.splparts.com/