Is there any place that shows how to get these? or are they just random?
Random. Best way to increase odds of finding item for your class is to get in 4 man groups of your class. Of course don't expect other Wizards to just give your their Wand of Woh... LMAO!
Gambling for it is extremely difficult as it costs 15 shards per roll which is very expensive. By the time you get a Wand of Woh through gambling you would've made 3-4 characters T6 ready through that amount of shards used on gambling.
As far as I know the following Legendary items are in the "extremely rare" category:
Witching Hour (high DPS belt that has IAS and CHD)
Shard of Hate (multiproc 1H weapon)
Wand of Woh (Wizard weapon allows multiple casts of Exploding Blast)
Calamity (1h Xbow weapon that procs Mark of Death on opponents, upping your DPS substantially)
Kridershot (2H Bow makes Elemental Shot into a Hatred Generator)
Chanon Bolter (2H Xbow makes Spiked Taunt taunt lesser enemies to it)
Focus and Restraint (dual set rings that makes your Spender deal 50% more damage if at full resource)
Little Rogue and Slanderer (dual set weapons, gives you attack speed for every resource spent for a total of 30% attack speed. One of the rarest items in the game even among the rarest of rares)
Frostburn (Rarest gauntlet in the game, chilled enemies are frozen)
Rimeheart (1h weapon, frozen enemies have a 10% chance to be instantly shattered. This weapon is ridiculous overpowered and is getting severely nerfed in 2.1)
Furnace (2H weapon, enemies take 7% of their life as fire damage. Ridiculous over powered weapon that is getting nerfed in 2.1)
All of the immunity Amulets are very rare Legendaries with the Star of Azkaranth (gives fire immunity) being the single rarest item in the game
Ice Climbers (Boots that give you frozen immunity)
Tasker and Theo (gloves that makes your pet attack 50% faster)
Here's the loot percentage of items in the game:
http://diablo.incgamers.com/forums/threads/legendary-drop-rates-data-torment-drop.858143/