This legendary crafting material is hard farmable, because there is only one bounty, which occurs very rarely.
There is only one bounty, which is worth to farm it and this bounty is very very very rare, so I suggest to farm this legendary crafting material in a group.
Act 2 - Howling Plateau / Black Canyon Mines - Kill Razormouth
1. Start an adventure game as highest torment level as possible.
2. Look for the act2 bounty Kill Razormouth (Howling Plateau).
3. If you found it, then invite your friends.
4. Take the waypoint to the Howling Plateau and run north to the Black Canyon Mines, where you will find Razormouth.
5. Kill him.
Good look finding this bounty.