Poké Doll - From a Youngster after showing him a Pokémon that is 8" (20 cm) or smaller (once per day)
Fluffy Tail - From a Lass after showing her a Pokémon that is 17' (5 m) or bigger (once per day)
Metronome - From the pianist in the northeastern house after showing her a Kricketot and a Whismur
Gracidea - Gift from the girl inside the Pokémon Center if she is shown a fateful encounter Shaymin.
Heart Scale - From a woman in the house south of the market, if the player has a Pokémon knowing the asked TM move (repeatable once per day)
Expert Belt - Received from a man in black at the market, if the player has a Pokémon that is level 30 or higher
Net Ball x5 - Pilot behind the counter in the Cargo Service building, for bringing him a Flying- or Psychic-type Pokémon
Exp. Share - Gift from chairman of the Pokémon Fan Club, if Pokémon shown has gained between 25 and 49 levels since it was met in a Generation V game
Cleanse Tag - Gift from chairman of the Pokémon Fan Club, if Pokémon shown has gained between 50 and 98 levels since it was met in a Generation V game
King's Rock - Gift from chairman of the Pokémon Fan Club, if Pokémon shown has gained 99 levels since it was met in a Generation V game
Shock/Burn/Douse/Chill Drive - Show Genesect to Scientist Dudley inside (Given drive(s) will vary depending on version)
Reveal Glass - Obtained from a man near Landorus's shrine if a Therian Forme Landorus is brought there
Black Sludge - After showing an NPC a Pokémon holding a Poison Barb
Dawn Stone - Gift from a Lady east of Youngster Mikey for showing her a Pokémon with Solid Rock.
Swift Wing ×1 or ×5 - From male Ace Trainer in a house near Lady Lina if he is shown a Pokémon with an Attack stat equal to (gives five) or higher (gives one) than what he requests. Can be repeated daily, with request values changing every time.
Gone through all the places, routes and cites. can't find more. Also, Arcazeus's Soothe Bell one apparently only applies to BW not B2W2 (Checked on Bulbapedia)
Also I didn't include people who want to trade with you/people who give you items after you have seen a certain amount of Pokemon, rather than a specific Pokemon.
Anyone feel free to comment any I forgot, and I'll double check it