This guide will tell you where you can farm for Gnashing Payara and Astral Twilight in Warframe.
Both Gnashing Payara and Astral Twilight can drop as a Rotation reward after completing the spy missions on Lua.
To get a rotation from any spy mission, you will need to get 1 of the 3 Vaults. Once you crack on the vault, you can let the other two be destroyed if you want to. The mods have the lowest drop rates from the items in its reward pool.