Black Belt (Fighting) - Poni Plains (dropped after Hariyama ambush encounters)
Held by wild Makuhita (5% chance)
Black Glasses (Dark) - Haina Desert (sometimes dropped after Krokorok ambush encounters)
Held by wild Krokorok (5% chance)
Hano Beach
Charcoal (Fire) - Held by wild Torkoal and wild Turtonator (Sun) (5% chance each)
Wela Volcano Park
Dragon Fang (Dragon) - Konikoni City
Hard Stone (Rock) - Festival Plaza (Haunted houses)
Poké Pelago (Isle Aphun - all paths)
Held by wild Roggenrola and wild Boldore (5% chance each)
Ten Carat Hill
Magnet (Electric) - Held by wild Nosepass (5% chance)
Metal Coat (Steel) - Held by wild Magnemite, wild Skarmory, and wild Beldum (5% chance each)
Miracle Seed (Grass) - Held by wild Fomantis (5% chance)
Route 8
Mystic Water - Held by wild Goldeen, wild Seaking, wild Dewpider, and wild Araquanid (5% chance each), wild Lapras, wild Castform (100% chance each)
Paniola Ranch
Never-Melt Ice (Ice) - Held by wild Vanillite and wild Vanillish (5% chance each)
Seaward Cave
Poison Barb (Poison) - Hano Beach (sometimes dropped after Tentacool ambush encounters)
Held by wild Tentacool, wild Tentacruel, wild Roselia, wild Venipede, and wild Mareanie (5% chance each)
Melemele Meadow
Sharp Beak (Flying) - Sometimes dropped after ambush encounters with Spearow and Fearow (Routes 3 and 10, Poni Plains)
Held by wild Spearow and wild Fearow (5% chance each)
Route 3
Silk Scarf (Normal) - Held by wild Trubbish (5% chance) and Garbodor (50% chance)SM
Hau'oli City
Silver Powder (Bug) - Berry fields
Soft Sand (Ground) - Melemele Sea
Spell Tag (Ghost) - Hano Beach (sometimes dropped after Sandygast ambush encounters)
Memorial Hill
Twisted Spoon (Psychic) - Held by wild Abra (5% chance)
Malie City Outer Cape
Soul Dew (Psychic/Dragon, Lati@s only) - Aether Paradise (Secret Lab A; from an Aether Foundation Employee)
Adamant Orb (Dragon/Steel, Dialga only) - Hau'oli City Mall
Lustrous Orb (Dragon/Water, Palkia only) - Hau'oli City Mall
Griseous Orb (Dragon/Ghost, Giratina only) - Hau'oli City Mall
Odd Incense (Psychic) - Konikoni City Incense stall
Rock Incense (Rock) - Konikoni City Incense stall
Rose Incense (Grass) - Konikoni City Incense stall
Sea Incense (Water) - Konikoni City Incense stall
Wave Incense (Water) - Konikoni City Incense stall
Draco Plate (Dragon) - Hau'oli Shopping District
Dread Plate (Dark) - Hau'oli City Mall
Earth Plate (Ground) - Route 1
Fist Plate (Fighting) - Hau'oli City Mall
Flame Plate (Fire) - Route 1 (Trainer's School)
Icicle Plate (Ice) - Hau'oli Beachfront
Insect Plate (Bug) - Hau'oli City Shopping District
Iron Plate (Steel) - Hau'oli City Mall
Meadow Plate (Grass) - Hau'oli City Shopping District
Mind Plate (Psychic) - Route 1
Pixie Plate (Fairy) - Route 8
Sky Plate (Flying) - Hau'oli City Mall
Splash Plate (Water) - Hau'oli City Mall
Spooky Plate (Ghost) - Hau'oli Cemetery
Stone Plate (Rock) - Hau'oli City Mall
Toxic Plate (Poison) - Hau'oli City Shopping District
Zap Plate (Electric) - Hau'oli City Mall
Normal Gem (Normal) - Route 15
There are way too many sources for me to list them all. If you want any specific fact verified, however, I can link the source I used.
Hope I helped!
