Hi, the MK-R head for the Z3100, and it's related R ink cart, were the first HP 70 series items discontinued by HP because the Z3100 was the only printer that used them. So they're hard to find.
Depending on what's wrong with your current MK-R head it may be possible to recover it. A common problem with all HP 70 heads is they get air in them and no longer print correctly, and can ultimately fail from overheating. If the head hasn't yet failed you can purge the air and re-prime them with ink and continue using them.