Any PSU that can do at least 1500mA over 9v DC and has a 5.5 x 2.5mm socket should be fine, something like this should do, just make sure you set the polarity to centre positive.
You'll probably be able to set it to 7.5v as well as I think the original adapter that comes with the Twin is 7.6v, when I briefly owned a Twin I left it at 9v though and it was fine.
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