Instructions
Go to any area you have a high chance of encountering a Rathian or Rathalos. The Forest and Hills, Volcano, Desert and Deserted Island are all common habitats for both creatures.
Bring your most powerful fighting equipment. Rathian and Rathalos are notoriously strong, so don't let yourself get caught off-guard. Both creatures tend to fly around during battle, so you may want to consider using a ranged weapon to make the fight go faster.
Kill the Rathian or Rathalos and carve it as you would any other monster to reap your rewards. Your chances of getting a Flame Sac are quite low --between 22 and 25 percent -- so repeat the process of finding and battling Rathian and Rathalos until you get one.