These are needed to upgrade talismans. 50 are required to evolve a cosmic talisman for the first time. Best farmed at Crucible of Pain or Power Slave (hard/madness) at the Kingdom of the Sands Level.
Another viable option for farming is the 3rd level of the Brave New World (any of the three).
By far the hardest of the 3 colorless runes to collect as it requires 50 in order to upgrade a cosmic talisman. The second evolution costs more but an average player rarely has the chance to do so.
Tip: never trade them for 3 star evo runes if possible as 3 stars will constantly drop at higher levels and will pile up on their own.