if i were you i would head up to relekka and telegrab the runes on that small island by the rock crabs. :D Edit: i have heard you make a profit by doing this too =]
every one should see this awesome guide!! it is helping me immensly, i have to make 50k bows so yeah =]. oh and the way i find to be the fastest to get AND make b strings is seers. pick the flax and spin then bank. repeat.