if it is the nylon bush at the bottom of the shifter, nissan sell them for about 20c. they are identical to r31 skyline/d21 navara and most dealers have the part in stock
I've had to replace 2 over the years, one on the gtr, one on my navara.
however....the nylon bush just goes into the selector, when it is missing you should be still be able to find all the gears, the shifter will just move around heaps. If you can't select a gear maybe the bush is jammed somewhere? In both cases for me the bush must have fallen to the bottom of the box because I've had no problems since