Where to find it

In each game: where the item shows up once (on the ground, as a gift, as a reward) and where you can always get it (shops, wild Pokémon holding it, prizes).

1. Gold / Silver · Crystal

   Always

   • Held by wild Chansey (2% chance)

2. Ruby / Sapphire · Emerald

   Always

   • Trade

3. FireRed / LeafGreen

   Always

   • Held by wild Chansey (5% chance)

4. Colosseum

   Always

   • Trade

5. XD: Gale of Darkness

   Once

   • Agate Village

6. Diamond / Pearl · Platinum

   Always

   • Held by wild Chansey (5% chance)

7. HeartGold / SoulSilver

   Always

   • Held by wild Chansey (5% chance)

8. Black / White

   Once

   • Chargestone Cave

   Always

   • Route 13 (treasure hunter)
   • Held by wild Happiny (5% chance) (W)

9. Black 2 / White 2

   Once

   • Celestial Tower

10. X / Y

    Once

    • Coumarine City

    Always

    • Held by wild Chansey (5% chance)

11. Omega Ruby / Alpha Sapphire

    Always

    • Held by wild Pelipper and wild Happiny (5% chance each)

12. Sun / Moon · Ultra Sun / Ultra Moon

    Once

    • Route 1 Hau'oli Outskirts

    Always

    • Held by wild Blissey (5% chance) (USUM)

13. Sword / Shield

    Once

    • Hulbury

    Always

    • Easter 2020 Wild Area News Max Raid Battle reward (April 10-15, 2020)

14. Isle of Armor

    Always

    • Max Raid Battles (3★, 4★, 5★ Chansey and Blissey raids)
    • Held by wild Blissey (5% chance)

15. Crown Tundra

    Always

    • Snowslide Slope Watt Trader (Highlight Item, 5,980 W)

16. Brilliant Diamond / Shining Pearl

    Once

    • Grand Underground Icy Cave

    Always

    • Battle Park (77 BP)

17. Scarlet / Violet

    Once

    • From Jacq before challenging the sixth Gym

18. Indigo Disk

    Always

    • Item Printer

19. Legends: Z-A

    Once

    • Vert District Pokémon Center (reward for completing Side Mission 109: "Wondrous Self-Healing Pokémon")

Source: Bulbapedia (CC BY-NC-SA)