That is not something that will be easy to find. I have had ones made for my Client's in the past when they have had a tricky situation.

Would you possibly be able to buy two, cut a piece off the second to get the length you need and stitch them together? You could then iron on a piece of ribbon or trim to hide the seam. If you're not a sewer, it's a very simple thing for a seamstress to do. I did find this on Amazon. It's longer than you need but could easily be hemmed.

You could buy an extra long curtain and get it hemmed to size. Do Internet search for "extra long" shower curtains.

If the current rod is a tension rod, lower it. Or buy another short tension rod and place it below this one. Then buy a shorter shower curtain. Not sure this would work for you, but thought I'd mention it. Good luck!