None of the fabric stores that I've been to sell mohair . . . I've always ordered it online from a mohair supplier. Our sponsor, Intercal, has a wonderful variety of mohairs in different lengths, textures, colors, prices. Just go to the top of this page and click on the 'Intercal' button at the top, right hand side. They have a very friendly staff and will help you pick what you're looking for. They're in California and shipping doesn't take long at all.
I hope it's okay to mention this, but there is also CR's Crafts, which is an online store. They have a gob of fabric and they're pretty reasonable, but I can't buy fabric online because as I can't see it, I have to feel it and they haven't found a way to do that over a computer, lol.