You can get them from high rank Rathalos/Rathian
High rank being Hr 31+
If your unsure you should check the Wiki
=)
You can get Inferno Sacs from high rank Rathian and high rank Rathalos. High Rank Quests are available after you beat the Jhen Mohran Urgent Quest and become lvl 31.
inferno
If you have more than one inferno, you have infernos.
Chimchar evolves a level 14 into Inferno.
A smouldering inferno.
inferno flame claw
Air Inferno happened in 1990.
Air Inferno was created in 1990.
Danny Inferno was born in 1977.
The Glass Inferno was created in 1974.
The ISBN of The Glass Inferno is 0890839840.
Phantom of Inferno happened in 360.