Your best bet is to kill lightning mobs in Muramite Proving Grounds, or Anguish (instance), even though the bracer is only about a 1% drop rate.
If you prefer to wait for mob spawns instead of roaming the zone, here are 3 places in MPG where there is a cluster of 3 lightning warrior mobs:
-1200, 2470 -- to 6-way, then head to northwest curlicue
-750, -2550 -- to 6-way, then head to northeast curlicue
-4180, -1840 -- to 6-way, then southeast to Stormer's room at the end, which is by Realms trial (Ingenuity / Adaptation)