This legendary crafting material is easy farmable, because of an 100% obtainable rare spawn.
Easiest method is to farm the Rare Monster Dragus in Stonefort, but there are also some other bounties, where you can have a look for.
Rare Monster Dragus
1. Start an adventure game as highest torment level as possible.
2. Take the waypoint to Stonefort.
3. Go the stairs up and there you will find Dragus.
4. Kill Dragus and repeat or do other bountys for your legendary crafting material hunt.
Act 3 - Skycrown Battlement - Thromp the Breaker
Act 3 - Stonefort - Kill Bricktop
Act 3 - The Keep Depths Lev 2 - The Crusher