They aint out there. Well, not off the shelf anyway... You can order various strings from early music specialists but they are expensive. Physics says that a strings pitch depends on it's tension, length and most importantly in this case, mass per unit length. To keep a standard pitch range or a usable one you need metal windings to add mass and reduce the diameter of the string while keeping it's elasticity. That is why before winding technology was available you had instruments such as the theorbo which used long strings to give the lower pitch range. When Catlines and rope strings arrived to give flexibility in the string it improved things a bit. It wasn't until wound strings became available that fixed string length instruments with the playable pitch range we take for granted today became possible. Developments in string technology was one of the driving factors in instrument form and function at the time. Still is today
What project are you investigating? I maybe able to point you to some specific help.