Why do you want one? They don't make/add any power. I don't know if they still make these but the design chaos spacers are the only ones I remember making one for the 1.8 sentra. Might want to search google a little harder but you can try this site first.
Edit: forgot about these guys. Might as well spend a little bit more and get a complete kit that helps keep the intake manifold and throttle body cool. At least this will actually help make a few ponies unlike the above spacer.