This is the complete list of heads for Salvador in Borderlands 2 and how to get them:
Head Name: Tree Puncher
How to get Salvador Tree Puncher head: Drop - Creepers in Caustic Caverns
Head Name: Boatmurderer
How to get Salvador Boatmurderer head: Drop - Terramorphous
Head Name: A Sense of Dread
How to get Salvador A Sense of Dread head: Mission - Symbiosis
Head Name: Marshall Mustache
How to get Salvador Marshall Mustache head: Drop - Geary
Head Name: Rasta-nefarious
How to get Salvador Rasta-nefarious head: Drop - Henry
Head Name: Bowler Badass
How to get Salvador Bowler Badass head: Mission - In Memoriam
Head Name: The Baron
How to get Salvador The Baron head: Drop - BNK3R
Head Name: Salvador
How to get Salvador Salvador head: Default - Default Head
Head Name: Handsome Jack's Mask
How to get Salvador Handsome Jack's Mask head: Drop - Handsome Jack
Head Name: Breaking Bald
How to get Salvador Breaking Bald head: Default - Default Head
Head Name: Vault Veteran: Sledgehammer
How to get Salvador Vault Veteran: Sledgehammer head: Other - Completed Borderlands 1
Head Name: Special Edition:Private Eyes
How to get Salvador Special Edition:Private Eyes head: Collector's - Collector's Edition
Head Name: Superduperfly
How to get Salvador Superduperfly head: Drop - Vermivorous
Head Name: Top Card
How to get Salvador Top Card head: Challenge - Pay the Bills
Head Name: Wizened Warrior
How to get Salvador Wizened Warrior head: Drop - Badass Enemies
Head Name: Pistola
How to get Salvador Pistola head: Mission - Statusque
Head Name: Gearhead
How to get Salvador Gearhead head: Default - Default Head
Head Name: Not-So-Jolly-Roger (Pirate Head)
How to get Salvador Not-So-Jolly-Roger head: Buy and complete Captain Scarlett and Her Pirate's Booty DLC
Head Name: Nutz And Boltz
How to get Salvador Nutz And Boltz head: Buy and complete Mister Torgue's Campaign of Carnage DLC
Head Name: Mustache of Legend
How to get Salvador Mustache of Legend head: Buy and complete Sir Hammerlocks Big Game Hunt DLC