Power supply doesn't seem to be available in USA
Yes, my search also could not find any replacement power supplies in the USA (except for one USA seller on Amazon with an inflated price ($250) and not a great seller rating).
You likely won't be able to read the drives in another NAS (even a ReadyNAS v2) and particularly if in a RAID setup, but if you don't have other backup of the data, getting a replacement PS from the UK may still be an option to consider.
Otherwise if you have the data backed up, I would probably replace it with a Synology NAS which I've experienced to be reliable and I do like the OS software (but I do only have 5 + years on my personal unit).
Steve