Wound String for Octave?

At 19" VSL I find that 0.20" ones pop before reaching d'. The smallest I have found are 0.18" ones that can get to d' but are just on the verge of popping.

Anyone know of a source for wound strings thinner than 0.18"? I am building a 19" VSL octave dulcimer for a guy who wants a wound base string.

It is looking like he may need to settle for a plain bass string.