Stupid question how do i get Monster essence and thunder sacs and perhaps a mission that gives me both

[The_True_Raven]

i'm probably stupid but i'm having trouble finding how to get the monster essence and thunder sacs i need like 2 of both to finish astalos armor that looks really cool

[JPM岩]

Monster Essence is from either Rakna, rarely volvidon, and altaroths (giant ants).
Lightning Sacs are from tobi and khezu in MR.