From Guild Wars 2 Wiki
Arcane Slivers are PvP crafting materials and represent the rarest of the three arcane tokens. They are used in recipes at the Mystic Forge to create new PvP rewards. As they are rare they will generally create the most visually impressive items of a specific rank. They have chance to be salvaged when using a PvP Salvage Kit on rare PvP weapons and armor, found in reward chests.“Used by the Mystic Forge to produce PvP items.
— In-game description
It is assumed that gold reward chests have the highest chance to drop rare items (and thus salvage into slivers), however research into drop rates is still ongoing.