From what materials can you make paint brushes?

My father loves to paint, and I love to make things with my wood lathe. I thought I would make him a few brushes. I know how to make a handle and ferrule. My question is how do I get fibers to make the strands of the brush? What are some of the natural sources, and some of the artificial? I suppose I could just go to Home Depot and buy a house painter's brush for bristles, would that work?

Now, I live in horse country, so I am sure I can talk some friends at church into letting me have a few strands of a horse's mane. Are there any other recommendations?

I made him a proportional divider using walnut sap and heartwood, but I don't know if he uses it very much. He may not use these brushes, but I'd like to make him something pretty at least.