I have been unable to locate a second AC adapter for My Cloud EX4100 in WD Store. I am a bit amazed that WD doesn’t offer power supplies to support their units.
You can purchase aftermarket power supplies on Amazon or eBay, but this may void your warranty.
I found a few offerings, but unsure which would be correct as each has a different adapter plug size:
Adapter Plug Size: 5.5 x 1.7mm
Adapter Plug Size: 5.5 x 2.5mm
Here is another one which looks to be identical in description, but not sure what the adapter plug size is:
Hope this helps!