So, the photo isn’t showing in your post — but — I thought Roosbeck _was_ also Pakistani?
For NN strings, Northshore Strings or Markwood Heavenly Strings. Dusty would only have strings specific to their own harps. For a wound string, especially on a zither tuning pin instrument (if that one is such) the winding has to be exactly the right length to come over the bridge pin when at pitch, but not interfere with the tuning pin because the windings usually won’t go through those holes.
You should also be considering _why_ that string broke. It is not normal for NNs to frequently break unless: 1) there is a flaw in the string, 2) there is a rough spot or sharp edge where it passes over, 3) the scaling for the instrument is too long, or 4) the wrong string was installed at some point. I would surely be looking for the cause before just replacing a string.
Also, these strings will be custom made. Getting a set of them is more usual and cost-effecting.