I think you can still order the gears from Nissan direct.
Its been a while since I ordered any, but it was an option.
If you can save a few more £££'s I'd order a Quaife 3rd gear set.
Granted its more money, but it'll be super strong.
Just make sure you order the semi helical version. Not the full straight cut gear as it won't work with the rest of the gears!