28215-5m000
Electrical, Body Electrical, Antenna & Radio, Antenna Mast, Sentra, 200SX, All
List: $29.23
(I read in Schludwillers' FAQ that Ratshack has a whippy rubber antenna mast with adapter for the X, but my local Ratshack told me yesterday that they no longer stock automotive audio equipment)...
Update: My local Autozone has the whippy short radio mast, Meira RoadWorks Replacement Antenna Rubber Mast, part # AW-RM1R, with the 4 adapters.
The adapter fits quite well on my 2002, goes all the way in, and is not loose at all. I do like the idea of the O-ring though, I'm going to look for one this morning.