Finding a 260Z box isn't easy, but that do have the 'better' factory ratio's.
However if your having a hard time finding any Zed box, my recommendation is a R32 Skyline RB20det gearbox with the L series bellhouseing fitted.
These are the strongest factory 71 series box's - only downside is the ratio's aren't quite as close as the 260 box's.
It all depends on your application of coarse...