It is my understanding that while the technology Apple use for their PCIe SSD cards is the same as for example standard Samsung SSD drives, in fact Samsung make the for Apple the actual connector is subtly different. As such a standard Samsung PCIe M2 SSD would not fit.
This means normally you can only fit a genuine Apple one. However the following is a compatible i.e. Apple connector style one. According to iFixit the Mac mini 2014 uses the same style SSD as the MacBook Air mid 2013 model. See https://www.ifixit.com/Teardown/Mac+Mini+Late+2014+Teardown/30410
I could not find a reseller specifically listing an upgrade for the Mac mini 2014 or the MacBook Air 2013, however you could try contacting these two companies who do upgrades for other models and might simply not yet have updated their websites.
See http://www.transcend-info.com/apple/jetdrive/
and http://eshop.macsales.com/shop/SSD/OWC