There are no aftermarket replacements that I'm aware of for the dual-connector roof antennas on the gen2 SL's. The best price I know about is NissanPartsDeal at $123.85:
https://www.nissanpartsdeal.com/parts/n ... 4ba0b.html
However, if the antenna works well and the only problem is the rubber gasket;
1) Grab a good gasket from a JY, or
2) Permanently seal the existing gasket with a circle of RTV around both sides of the inner sealing rings. If you film the roof with WD40 before screwing it back down, the RTV will seal but won't adhere to the paint, or
3) Cut your own gasket from a sheet of outdoor foam or vinyl and seal it as above.