This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for Selfish Bulma side quest.
Time frame: From Evil Emperor Frieza - Episode 4: Gohan's Latent Power - Episode 4: The Fearsome Emperor
Reward: 9,314 EXP, Brain Bread, Mega Mass Lvl 2
Where to start: South-Eastern Islands, as Gohan.
You start the mission by talking with Bulma. Your task is to bring 3 iron ore and 3 Alien Whitefish Fin Muscle.
Iron can be extracted from crystals on the planet.
You will catch fish by heading to the nearby fishing spot. Return to the quest giver with the collected items.