A Gaffers box can be one of the most important things that a photographer can place in his vehicle for location shots or housed somewhere in a studio. While traditionally the word "Gaffer" has applied to electrical supplies, it can much more. Although your camera specific lenses and supplies will often be housed in one bag, materials in your Gaffers kit are typically much more general in nature. Although the exact content will vary from person to another, below are some ideas as where you may look to pick them up.
2. "A" clamps (Many uses including holding backdrops to stands) - Plastic or metal
3. Leatherman "type" tool. This are multifunction tools that often have pliers, screwdrivers, knives, etc. all integrated into one tool.
4. Measuring tape (Many uses)
5. Twist ties/Cable Ties (Single use, reusable,) MANY uses
6. Scissors (Many uses)
7. SuperClamps or Grip Heads (Useful for attaching things to lightstands)
8. Absorbent and or Wet towels (For surface cleanups and dirty hands)
9. Multi-Plug electrical outlet/extension cord
10. Weight Bags (to weigh down stands outdoors or heavy traffic areas)
Although technically not part of a Gaffers kit, consider the Vagabond Mini from Paul Buff, Inc. In many places where electrical outlets may be limited this device can be valuable ( $239)