Truesilver is a rare spawn of Mithril. Ergo, find mithril and you will find Truesilver. There's no real reason for this, as Mithril will take you into Small Thorium before it turns grey. But if you want to powerlevel your mining, yes, find any place with a ton of mithril.
Edit: In addition to what the above proster suggested, you can also look around Tanaris, at any rocky area (mostly make a circle around the outside of it), or Thousand Needles, where there are few mithril spawns (and they are decently spread out), but the enemies are very low levels.
Edited, Sat Feb 25 10:43:44 2006 by LockeColeMA