I don't see extra power cables for the i5 2011 Mac Mini in the Apple Store Accessory section.
But this power cable looks a lot like black ones I've seen that go between a power brick for a Lenovo laptop and the outlet. Is this the very same cable?
The Mac Mini uses the standard IEC 60320 C-7 female connector, take a look here for useful reference -
C-7’s are common and available everywhere (e.g. Amazon, Target, Best Buy, etc.)