Unless you are obsessed with authenticity you should just buy the Radio Shack #273-1771. Its 9V 1000mA, and you just pick whichever tip you need from the shelf when you buy it. Its just about the only aftermarket power supply you can easily get nowadays that will supply so much current. Also, its white, like the system, which not even the NEC one is.
BTW, there is a good chance that a genuine Duo R/RX power supply will just crap out on you a month later since they are all getting quite old at this point.