UPNetwork: Ultimate Pokémon Network

Just a simple listing of items on wild Pokemon. I recommend getting Zigzagoon and training it to level 29, when it learns Covet. Also note that I didn't include anyone from Battle Tower.

Berries

Chesto Berry (No.2): Kecleon, Whismur, Loudred, Spinda (rare)
Pecha Berry (No.3): Poochyena (common)
Rawst Berry (No.4): Numel, Vulpix (common)
Leppa Berry (No.6): Skitty (rare)
Oran Berry (No.8): Zigzagoon, Pikachu (common), Linoone (rare)
Persim Berry (No.9): Girafarig (rare)
Sitrus Berry (No.10): Linoone (common)

Evolution Stones/Shards

Red Shard (Fire Stone): Corsola (very rare)
Green Shard (Leaf Stone): Relicanth (very rare)
Blue Shard (Water Stone): Clamperl (very rare)
Yellow Shard (Thunder Stone): Chinchou (very rare)
Sun Stone: Solrock (Ruby only) (extremely rare)
Moon Stone: Lunatone (Sapphire Only) (extremely rare)

Other Items

Stardust: Staryu (common)
Heart Scale: Luvdisc (common)
Everstone: Geodude, Graveler (rare)
Star Piece: Staryu (rare)
Nugget: Grimer (rare)
Smoke Ball: Koffing (rare)
Big Pearl: Gulpin (rare)
Dragon Scale: Horsea, Bagon (very rare)

Battle Items

Twistedspoon: Abra (rare)
Spell Tag: Shuppet, Dusclops, Banette, Duskull (rare)
Metal Coat: Magnemite, Magneton (rare)
Hard Stone: Aron, Lairon (rare)
Poison Barb: Cacnea, Roselia (rare)
Quick Claw: Sandshrew (rare)
Soft Sand: Trapinch (rare)
Sharp Beak: Doduo, Dodrio (rare)
Light Ball: Pikachu (rare)
King's Rock: Hariyama (extremely rare)

Trainer Held Items

Black Belt: Black Belt Nob (5th battle, on level 33 Machamp)
Dragon Fang: Dragon Tamer Nicolas (5th battle, on level 49 Shelgon)
Silver Powder: Bug Maniac Bradon (5th battle, on Masquerain)
Nugget: Held by Lady Cindy's Pokemon
Nugget: Held by Rich Boy Winston's Pokemon