Where can I get NM cable clamps for a metal box?
Halex 3/8 in. 2-Piece Clamp Connectors




  Product Description: Use the Halex 3/8 in. 2-Piece Clamp Connectors (5-Pack) to connect #14 - #10 non-metallic sheathed cable to steel
  
  outlet boxes or other metal enclosures. They are compatible with
  standard 1/2 in. knockouts.
  
  
  Secure #14 - #10 non-metallic sheathed cable to steel outlet boxes or other metal enclosures
  Zinc-plated steel resists corrosion
  Fits standard 1/2 in. knockouts
  For indoor use only
  Designed for easy installation
  MFG Brand Name : Halex
  MFG Model # : 26510
  MFG Part # : 26510
  


Or as @Aaron suggested

Halex 3/8 in. Twin Screw Non-Metallic Cable Clamp Connectors




  Product Description:
  
  Twin Screw Non-Metallic Cable Clamp Connectors to join non-metallic
  sheathed cable and flexible cord to steel outlet boxes or other metal
  enclosures. A reversible locknut cuts through protective coatings on
  boxes and enclosures to preserve grounding.
  
  
  Connect non-metallic sheathed cable and flexible cord to steel outlet boxes or other metal enclosures
  Body and locknut are die-cast of high-strength zinc alloy
  Zinc-plated steel clamp and screws
  Reversible locknut cuts through protective coatings on boxes and enclosures to assure ground continuity
  Fits 1/2" knockout
  For indoor use only
  Screws compatible with Phillips, slotted and square screwdrivers
  Resists corrosion
  UL and CSA safety listings
  MFG Brand Name : Halex
  MFG Model # : 20511
  MFG Part # : 20511