When I taught guitar, I used to order them a carton at a time(30 copies) the from publisher(Sing Out Publications--1.888.SING-OUT)--they worked out to be just about half-price-used book sellers tend to price them at half cover price(sometimes more, because people want them), so you will have new books for about the price of used ones--

They used to give a good discount on smaller quantities, as well. One hint, get the money from your people up front, or you will end up chasing people to the ends of the earth to collect your eight bucks--