Most of them are taken from metal chest or from fallen zombies. Here the 23 legendary weapons: (Direct quotes from the wiki)
Assassins greed: metal chests in the city of Moresby, and occasionally in metal chests in the Laboratory area, as well as looted from a fallen zombie.
Ban: metal chests or on dead enemies
Banisher: metal chests or obtained from a fallen zombie.
Bloodletter: bought from Wayne at the Paradise Survival Camp
Blunt Speaker: metal chests or as a random drop
Boucer's Pal: can be found in the hotel
Bushmaster: random drops or weapon chests
Crowd Pleasure: random drops and in metal chests.
Defender of the Motherland: rare drop that can be found on dead enemies or in metal chests
Dragon Knife: bought from Wayne at the Paradise Survival Camp
The Eviscerator is not found in any precise location. It is a random drop from zombies across the Banoi Island and has a small chance to be looted from metal chests.
There have been reports of players acquiring it as early as during the quest "Light My Fire" in the Resort area and all the way to the Jungle chapters.
It has been reported to have been found in the beach area lodged in a walkers head next to a corpse under a yellow and white umbrella
Gabriel's Sledgehammer: completing her quest "Bury the Dead" (Level when collecting reward make weapons's stats vary)
Home Run: dead enemies or be found in metal chests with small chance of spawning in random location
Italian plumber: locked chest
M.C: dead enemies or in metal chests
Master Chef: metal chests and occasionally looted from a fallen zombie.
McCall: dead enemies or in metal chests with small chance of spawning in random location
Pick of Destiny: quest "Fallen Angel"
Tijuana machete: quest "Where is my pineapple?" (Level when collecting reward make weapons's stats vary)
Two Parter: dead enemies or in metal chests with small chance of spawning in random location
Viking of the south: metal chests throughout the Banoi Island or obtained from a fallen zombie.
Zed's Demise: quest "The Butcher of Banoi", (Level when collecting reward make weapons's stats vary)
So Two Parter, McCall and Home Run can be found by spawn with very small chance, at random location, but also by metal chest and killing zombie/enemies