Where can I buy a spare power cable for a i5 Mac Mini? 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?
Provided it connects fine you should be ok. It uses a standard power plug with this design. http://support.apple.com/kb/HT4806 

Otherwise any sort of Apple Service Provider or http://www.mcmelectronics.com/product/23-190