Nismo don't make any larger than 480cc item for the R34 RB25det neo motor.
I would suggest buying the Nismo 480cc as above ^^^ and the bumping up the fuel pressure (with a FP regulator) twin spray pattern and high impedence, meaning less lag time for fuel to be injected, something like 0.01sec
Otherwise the other options that you have available are:
Highflow standard injectors - approx. $150 each (which aren't the best, can f!ck up spray pattern, impedence etc)
Sard injectors - 600cc or 800cc - they require a collar to be attached and aren't straight bolt in approx. $1200-1400
Blitz injectors - pretty much the same as sard injectors
HKS injectos - 650cc-850cc, requires hks fuel rail - all up approx. $1400