Truesilver is a rare spawn of mithril and thorium nodes - you will occassional find it anywhere one of those nodes spawn, but you can't really farm for it except by farming all the mithril and thorium nodes you find and hoping one will occassionally respawn as truesilver.
Un'goro crater is my favorite spot for thorium, and there is sufficient mithril there that making several passes should net you at least some truesilver. There are also rich thorium veins in teh southwestern section which will yield the occasional arcane crystal for a burst of cash, once your mining reaches that level. If your character is a little low to safely run through Un'Goro, try Tanaris or Searing Gorge.