The silver feather is a quest item used in the Eagles' Peak quest. It is found on a pedestal inside some caves under Eagles' Peak. The player must use the silver feather along with a bronze feather and a golden feather in order to open a locked door that leads deeper into the caves. The feather can be reobtained after the quest and multiple copies can be obtained by banking the duplicates.
For an exhaustive list of all known sources for this item, see here