There are some brush knots at aliexpress.com too. I have bought from each supplier that sells individual knots and have never had a problem. There is a wide variety of quality between manufacturers though so do some research before buying blindly.
Wet Shaving Products has an excellent selection of knots. They carry High Mountain White, Silvertip, 2-Band Finest, and other grades. I'm familiar with their 2-Band Finest and it's a great knot with backbone and very soft tips.