The original legendary weapon, the Lost Sword of El Patron, can theoretically be found on any enemy or ship. The most common and effective way to get one is to fight a high level boss, such as General Darkhart or Neban the Silent. It can also be worth trying to get one from high level enemies such as Davy Jones' crew (I got two of mine there).
Silver Freeze can also be found anywhere, but the most efficient way to get one is beyond a doubt from Foulberto Smasho, the ghost boss on Raven's Cove. He always drops throwing knives. Famed knives are relatively common, and Silver Freeze drops occasionally. This is easily the most easy to get.
The remaining five legendary cursed swords can only be found from ghosts on Raven's Cove (Excluding Foulberto) or from 'Davy Jones' crew on Tormenta.
Neither of these strategies guarantee a drop, and you may need a fair amount of patience to get any. There is no confirmed "fast" way to get one, as they all have low drop rates.
Some claim that servers go through "cycles" that decide whether famed or legendary weapons have a higher chance to drop, and that famed and legendary items therefore are easier to find if you change servers once in a while if your luck is down. This is however not confirmed.