my grandpa once year ago had a nice leather whip hanging on his wall, and he once took it down and let me play with it on the back porch. (amazing i didn't take an eye out!
)
this was my first enounter with a real whip. i was cracking it away, and after a minute, i noticed all these little fibers floating around in the air around me. i took up the end of the whip in my hand and looked at it, and found that there was a little cord tied to the end that was falling apart. it was a long time ago, but if my memory serves correct, then it was something like that really course, ugly, rough packing twine that they sell really cheap, but a more refined peice. it is a pale yellow, and really uneven along the length. you probably know what i mean. anyways, try some rough natural fibers, like you said, as these may have good results, even though you may have to switch them out often.
whipartist still has a webpage up, and you could check that out to see what he uses, though you most likely already have.