For experimental instruments or for those professionally made unique items, there is a very simple and easy resource for bow strings - be they for violin, huqin, or any kind of bowed, stringed instrument.

Dental floss.

What? Yes, that's what I said. Dental floss.

Hear me out here for just a mite.... while horse hair is widely used for this, it is the common belief that this is due to the ridged, or tooth-like quality, of these hairs - that it is this, which, when dragged across the instrument's strings, micro-plucks said strings to make the vibration, and thusly, the sound. For more older types of folk instruments - those that use no rosin - this is true.

However, for the more modern types, it is the rosin (refined tree resins, such as those from pines) which creates the sound. How so? Well, it is a somewhat sticky substance, being made from tree sap, and so it sticks to the strings of the instrument, pulling them, and letting go - then sticking again, pulling, and letting go - then sticking again, pulling, then letting go.... a jillion micro instances in a row, as you draw the bow across the instrument's strings. It is this that creates the vibration, and thusly, the sound. The ridged quality of horse hair is simply exceptionally good at holding the rosin onto the bowstrings.

But it should be noted, that yes, it actually *IS* the ridges of the horse hair that does this on bows that use no rosin, as in some bowed folk instruments.

Anywho, back to dental floss - it should be the non-flavoured variety, and cotton. The weave of this also serves quite well at holding rosin. As an experimental piece, or one you might utilize to bow some strings on a few projects as they are being made to check for sound, try this;
Take a wooden dowel (or willow shoot, or long twig/branchlet, if you are good at whittling and feeling a little adventurous), and cut a slot into both ends - take a few strands, 5-7 or so, of the dental floss, a couple inches shorter than the dowel, and knot at both ends. Bending the dowel carefully, so as not to break it, place each end of the floss bundle into the slotted ends of the dowel, so that the floss bundle's end knots hold it in place. Finally, rub some rosin onto the bow.
Based on this crude example, we can learn how and why a violin bow works the way it does.

To learn a bit more about bowed instruments (or even plucked strings), let's use a stick about 2 or 3 feet long (perhaps even a yard stick), an *E* electric guitar string - this is the thinnest string, the highest note string, on electric guitars, which can be gotten individually from most any music shop, for about 75 cents to little more than a dollar - and, a small balloon.

Yeah, that's right, a balloon.

Okay - wrap the free end (the end without the tiny metal ring tied in) of the guitar string around one end of the stick, tightly. Next, cut a notch, or slot, into the other end of the stick for the other end of the string to go into. The tiny metal ring tied into that end of the string should hold that into place.
Don't have this string be very tight, because you'll now want to inflate the small balloon, and place it between the stick and the string, wedging it into place - once the balloon is thusly wedged, and preferably more towards one end of the stick than the other, there should be some tension in the string. Most music artists would "tune" this prior to performing.

Now take the bow we made from the experiment before, and bow the string.

Not a bad quality of sound, eh?

See, this balloon is acting as a soundboard/resonating box - the vibrations from the string are being transferred into the balloon, which is merely an enclosed hollow, surrounded by a spherical diaphragm. In this case, the balloon is almost like a drum (even acting like an "eardrum", though somewhat in reverse, in it's transmuting vibrational data into sound), which is 100% drum head.

For another experiment, you can even get excellent results from closed-cell styrofoam - now, a block of this stuff can have the "stick" of the instrument pass through it.... now, place a "bridge" on the face of the block - that is, perhaps a thin square of wood, on edge, for the string to rest across, which will transfer the string's vibration into the block of styrofoam.
This will produce a good, amplified sound.

This author loves teaching music related topics. Chris W. could write all day about this stuff. Bless the music world.

Related Article:
Homemade Musical Instruments