Big Mushroom - Paras, Route 9
Chilan Berry - Rattata, Fortulose Manor
Stardust - Staryu, Rosecove Beach
Note that Rattatas with Chilan Berry and Paras with a Big Mushroom have a very small chance of appearing! (5%)
Once you find all three items, go back to her house and she will award you with an Epineshroom. You must after that go to Brimber City and give it to the Black Belt that is standing next to a large rock. He will smash it open and it will unveil a pathaway to Volcanion's cave. In it, you can encounter Volcanion and attempt to catch it.
Use a Pokémon with the ability 'Compound Eyes' to increase the chance of getting any of these items such a Joltik or Galvantula.
Remember to take the items off the Pokémon before giving them to her.