Apple uses a custom SSD blade connector for their SSD's. Both the connector assembly & the SSD's are hard to find (so far only Apple units available as spares or from mini's taken apart for the parts). And both are very expensive!
If you just got it see if you can return the unit and get the SSD model, then you can add in the HD as the SATA connector assembly is the same as the last model and a standard SATA III drive will work.