I buy horsetail hair for bows.
My bow hair is the best in the world. Here's a short list of some of my customers:
George Rubino, Bow Maker, Benoit Rolland, Bow maker, John Montgomery Violins, Serge Sioufi, Bow maker, Matt Wehling, Bow maker, Gianna Violins, Pier Bergeron, George Heinl & Sons, Nicholas Caraccio, Bow maker, Jon Vanderhorst, Brad Dorsey, Ted Herbert's Music, Renaissance Strings, Collins Violin Bows, Bridgewood & Neitzert, Schonau & Tardif Luthiers, Inc., Arte - Suibumn Leigh, Seoul, KR
I buy in small to medium-sized quantities to keep the supply fresh.