Truehide: Ashora - Near Ajuros (nexus area) around the pond where the tower is there are terror maws that drop truehide and are on a very fast respawn. This is my secret farm spot. Unfortunately, it is also a very farmed spot for everyone else.
Empyrean Silk: Steppes of Infinity near the two fortresses to the south (where the defense event happens) there are tons of mobs there, you can do a loop using the porters .
Karthite: Ardent Domain - Witches Pond but really anywhere in this zone.