The J Matic fluid is a fluid for life.
However if you prefer to change, then the old J Matic Fluid ( Nissan Matic J: Part Number KE908 99932 has been superseded by Nissan Matic S (J) Part Number 999MP MTS00P which is readily available from Nissan dealerships. On the rare occasion that they are out of stock, they will bring in from Amsterdam or worst case, Japan.
Also when swopping out the fluid, you will have to decide if you are carrying out a full change or a partial change, as the full change means emptying the torque converter and the oil cooling pipes and rad. etc.
Expect the fluids to cost North of £200.00
After market fluids are available but I personally would stick with oem.
To summarise: An unnecessary expense IMHO as it is a fluid for life on UK cars, however I have answered your original question.