Where can I get a HD replacement?

If you are upto it you can swap out the HD with either a SSHD or a SSD both will improve performance. As an example: Seagate laptop SSHD or Samsung 2.5 SSD

The rub here is this newer model supports a custom Apple SSD blade drive for the second drive. Presently the needed parts are not available to add it.

Review this IFIXIT guide: Mac Mini Late 2014 Hard Drive Replacement. It's not to hard if you take your time and don't force things you can do it.

If you really don't want to attempt it I would recommend you check out Apple's support site for a local authorized service center. As I'm sure you can find one close to you who can do it for you.