Okay, I'll help with some of this.
Legendaries
- Cobalion is at Route 13 at Level 45
- Virizion is at Route 11 at Level 45
- Terrakion is at Route 22 at Level 45
- Zekrom is at Dragonspiral Tower at Level 70 (Black 2)
- Reshiram is at Dragonspiral Tower at Level 70 (White 2)
- Kyurem is at Giant Chasm at Level 70
- Latios is at the Dreamyard at Level 68 (Black 2)
- Latias is at the Dreamyard at Level 68 (White 2)
- Uxie is at Nacrene Museum at Level 65
- Mesprit is on top of Celestial Tower at Level 65
- Azelf is at Route 23 at Level 65
- Regirock is at the Underground Ruins at Level 65
- Regice is at the Underground Ruins at Level 65 (White 2)
- Registeel is at the Underground Ruins at Level 65 (Black 2)
- Regigigas is at Twist Mountain at Level 68
- Cresselia is at Marvelous Bridge at Level 68
- Heatran is at Rebirth Mountain at Level 68
Gift
- N's Zorua is given to you at Driftveil City at Level 25 by a sage
- Eevee is given to you at Castelia City at Level 10 by Fennel (Hidden ability Adaptability)
- Deerling is given to you by the scientist at the weather institute at Level 30 (Hidden ability Serene Grace)
- A shiny Gible is given to you by Alder's grandson in Sangi Town at Level 1 after defeating him at Black City (Black 2)
- A shiny Dratini is given to you by Alder's grandson in Sangi Town at Level 1 after defeating him at White Forest (White 2)
- An egg containing a Happiny is given to you by a breeder
- A fossil is given to you in Twist Mountain daily by a worker and they can be resurrected into Omanyte, Kabuto, Lileep, Anorith, Aerodactyl, Cranidos, and Bastiodon depending on the fossil given
- A fossil is given at the Nacrene Museum by Lenora as congrats on defeat of the Elite Four, and can be resurrected into Archen or Tirtouga, depending on chosen fossil
Interactable
- Volcarona is found at Relic Castle at Level 35
- Mandibuzz can be found at Route 4 at Level 25 (Hidden Ability Weak Armor, Black 2) THURSDAYS ONLY
- Braviary can be found at Route 4 at Level 25 (Hidden Ability Defiant, White 2) MONDAYS ONLY
- Foongus can be found on Route 6 as Poke Balls at Level 29
- Jellicent occasionally appear at Undella Bay at Level 40 (Hidden Ability Damp)
- Crustle can appear at Seaside Cave at Level 42
- Shiny Haxorus can be found at Level 60 at the Nature Sanctuary at Mistralton City
Trade
- In Nacrene City, a Petilil @ Power Anklet, Timid Nature, Female, Own Tempo, Level 20, with Sleep Powder, Mega Drain, Synthesis, Magical Leaf is offered for a Cottonee (Black 2)
- In Nacrene City, a Cottonee @ Power Weight, Modest Nature, Female, Prankster, Level 20, with Stun Spore, Mega Drain, Cotton Spore, Magical Leaf is offered for a Petilil (White 2)
- In Route 7, a Gigalith @ Leppa Berry, Jolly Nature, Sturdy, Level 35, with Iron Defense, Smack Down, Power Gem, Rock Slide is offered for an Emolga
- In Seigaiha City, a Tangrowth @ Sitrus Berry, Jolly Nature, Chlorophyll, Level 45, with Natural Gift, Giga Drain, Slam, and Ancientpower is offered for a Mantine
- In Route 15, a Rotom @ Max Elixir, Gentle Nature, Levitate, Level 60 with Substitute, Hex, Electro Ball, and Charge is offered for a Ditto
- In Accumula Town, a Ambipom @ Sitrus Berry, Jolly Nature, Pickup, Level 40, with Nasty Plot, Agility, Fling, Double Hit is offered for an Excadrill
Hidden Grottos
Route 2
- Nidoran M/F (Hustle), Granbull (Rattled), Watchog (Analytic)
Route 3
- Lombre (Own Tempo), Manectric (Minus), Bibarel (Moody), Pachirisu (Volt Absorb), Zebstrika (Sap Sipper)
Route 5
- Liepard (Prankster), Mincinno (Skill Link), Foongus (Regenerator)
Route 6
- Woobat (Simple), Foongus (Regenerator)
Route 7
- Watchog (Analytic), Cubchoo (Rattled), Zangoose (Toxic Boost), Seviper (Infiltrator)
Route 9
- Muk (Poison Touch), Liepard (Prankster), Garbodor (Aftermath), Bouffalant (Soundproof)
Route 13
- Tangela (Regenerator), Spheal (Oblivious), Drifloon (Flare Boost), Skorupi (Keen Eye), Foongus (Regenerator)
Route 18
- Fearow (Sniper), Kingler (Sheer Force), Dragonite (Multiscale), Chatot (Big Pecks)
Route 22
- Pelliper (Rain Dish), Amoonguss (Regenerator), Mienfoo (Reckless)
Route 23
- Golduck (Swift Swim), Gligar (Immunity), Zangoose (Toxic Boost), Seviper (Infiltrator), Absol (Justified)
Sangi Ranch
- Marill (Sap Sipper), Dunsparce (Rattled), Herdier (Scrappy)
Pinwheel Forest
- Butterfree (Tinted Lens, White 2), Beedrill (Sniper, Black 2), Poliwhirl (Swift Swim), Murkrow (Prankster), Breloom (Technician), Hariyama (Sheer Force), Medicham (Telepathy), Bagon (Sheer Force), Amoonguss (Regenerator)
Lostlorn Forest
- Pinsir (Moxie), Heracross (Moxie), Combee (Hustle), Leavanny (Overcoat)
Giant Chasm
- Clefairy (Friend Guard), Ditto (Imposter), Sneasel (Pickpocket), Metang (Light Metal)
Abundant Shrine
- Vulpix (Drought), Golduck (Swift Swim), Swablu (Cloud Nine), Bronzor (Heavy Metal), Amoonguss (Regenerator)
Note: Eevee can be caught in any Hidden Grotto.