Brain Fungus: Whitespring Service Entrance. Look around the dumpsters. Also in Abandoned Waste Dump.
Fore Firecaps and Starlight Berries, spawn at Gauley Mine (or run from Artos Pharam) and follow the train tracks down to the Gauley Mine. They both spawn in copious amounts at both sides of the tracks.
Firecracker Berries, look North West around Arktop Pharma
Those looking for Carrot Flowers, go to Palace of the Winding Path. 80 carrot flowers in 5 minutes.
Mentats Crafting:
Brain fungus (2)
Carrot flower (1)
Firecap (1)
Berry Mentats Crafting:
Mentats (1)
Firecracker berry (1)
Starlight berries (1)
Brain Fungus (1)