How to Become a Legion Slaver or a Manhunter:
Caesar's Legion:

• Go to the "Offices of Slavery" in Cottonwood Cove. The Offices of Slavery is located in one of the cabins on the left of the cottonwood (left if you're looking towards the beach).
• Speak to Scribonius Libo Drusus. Get through his dialogue. Ask what it takes to be a slaver. Afterwards, you will have the option to purchase collars from him.
• A Radiant/Repeatable quest, "Opera Magna", can be taken granted by Scribonius. However, you must have above 25% total fame with the Legion.
  

• Head to Camp McCarran's main building (the one with the escalators). Standing in the center of the first floor, look for the nearest doors that lead to the runway. Once you exit through those doors, you will see a Manhunter standing in front of a door. Enter through that door.
• Speak to Gunnar, the dude with the face tattoos. Ask if you can join the Manhunters. You will need to have performed actions that benefit the NCR, such as:
  2a. Complete Three Card Bounty
  2b. Killing Motor-Runner
  2c. Discover the McCarran Spy (either the real one or the framed one)
  2d. Interrogate Silus (successfully)
  2e. Aid the Van Graffs in the Legion Ambush
  2f. Wipe out the Great Khans
  2g. Killing Caesar
  Each one of these actions is weighted differently. So it's best to do a few.
  
• Once you have "proven yourself", you will have the option to purchase collars from Gunnar.
• Gunnar doesn't provide a radiant/repeatable quest (yet...), but he does provide a quest for the player once they become a Manhunter.