As stated above, you will need to source it from a cruiser with a B engine.
In the Canadian market BJ42s and BJ60s had 5 speeds from 83 on. Not sure on the month of change over on that one. I don't know about the changeover in NZ (I assume thats where you are from)
If you get a 5 speed from a 60 series, you will be golden. If you get one from a 40 series, the shift tower will be in a different location (about 3" ahead) so you will either have to aquire a 60 series shift tower or modify (i.e cut) the floor.
Note, try and get the t/case with the H55F tranny as the 4 speed t/cases don't have the oiler for 5th gear. Also try to get the shifter linakge because it is different as well..
You can get a new H55F from Toyota but over here they are ~2500CDN
Hope that helps.
Daryl