duo r and rx uses standard 9v adaptor which is very common on ebay, radioshack, tandy ( presume they still exist), maplins, etc, (us snes and duo at 10v is much rarer, xrgb 2 at 14v is very rare), a standard multi adaptor (3,6, 9, 12 ) is fine or a fixed voltage one at 9v
the Mah/milliamps at 1a/1000mah, as long the replacement psu's milliamps is higher, ie1.3a , 1800mah, etc, then it's fine, the machine draws its required current, try avoid over double than required current, waste electricity and adaptor may get rather warm
if the op is in UK, most of asia and Europe, Australia, get replacement at 240v (step down is a waste of electricity in converting, thus original adaptor is rather pointless, this is not audiophile equipment thus no difference)
if in japan or us, the original is fine (though they are bulky 90's tech) but considering their age, could unreliable, as long it's 120v