short answer - mine mithril.
longer answer - truesilver is a rare spawn of mithril nodes, just as gold is a rare spawn of iron, and silver a rare spawn of tin. If you mine all the mithril you can find and keep returning to the same nodes over and over again, they will occasionally spawn as truesilver instead of mithril. Also, somewhat rarer, thorium veins will respawn as truesilver.
I don't know what your current character level is or what zones you are comfortable in, but the lowest level zone that seemed to have lots of mithril for me was the Badlands. There are a few nodes in Lethlor Ravine, and others in the various valleys along the outer rim of the zone. If you are somewhat higher, Un'goro crater is a good place for mithril along the northwestern quadrant and thorium along the southeastern quadrant. Check the maps for the exact locations of the spawn points.