Just gone through the pain of this.
The thread size (12) and pitch (metric fine, 1.25) is easy to get hold of, its the spline pattern and spline diameter thats the difficult bit, "modern" Nissan spline diameter is 14mm but the 240/260 needs 12.8mm.
Nismo do a set but they are not cheap, alternatively you can have the hubs drilled out to suit more available patterns but this is not without risks